Talos

Those are definitely mounts for the breeching ropes of the cannons. With having to clear the bench in front of the ports there, I wonder if they're more meant for the two-wheeled iron Marsilly-style carriages like you can see in this mislabeled gundeck photo of Santee. https://upload.wikimedia.org/wikipedia/commons/4/49/Gun_deck_USS_Richmond_LOC_4a14697v.jpeg(you can see the name Santee on the buckets). Those carriages make more sense for the stern anyway, since they allow you to shift around one or two guns easily without pivot rails or filling all four ports constantly. You can even store the guns normally out of there, so they don't take up space in officer country.
 
EDIT: You can find/download a bigger version of the Sabine gundeck (facing the bow, I might add) picture here. http://www.loc.gov/pictures/resource/det.4a14697/

It's certainly possible, of course, though I think I got them right, with the fourth main deck port being hidden by the curve of the stern. I point out the port by the bow of the stern boat being almost directly above, as well as the quarter gallery badge (probably a star) in the next space directly to the left, like on the modern stern of Constellation. http://l7.alamy.com/zooms/83f9395f348a497984c631fe8caedc81/uss-constellation-seen-from-the-stern-baltimore-d0w36x.jpg
 
It wouldn't make sense for the ports to function as quarter-gallery access. Not only would that be defeating a major reason for the stern shape (allowing fire over the rear quarters, eliminating a major blindspot  that men-of-war had,  but the quarter port is visible on all the plans of round-sterned ships with quarter galleries immediately forward of it. The Sabine plans actually shed some light there. They're missing quarter galleries and stern detail, so I overlaid them with the Brandywine's stern (which does have them) and they're an exact match. Rather than the quarter ports, they actually cover up the very last broadside gunport aft. You can see it in the heavy frigate comparison I threw together last year. 
 
You're right about Jamestown. The stern detail on the draught and on the real ship is amazing. Notice also in the picture that there are gunport covers fitted on the stern ports, hinged at the bottom and hanging horizontal. Her plan is so weird compared to the other super sloops of her construction program. Vertical, heavy stern, fitted with a spar deck from the start, and quite a bit larger than the rest.

She still has guns twenty years later. Notice they lowered the yards and they're resting on the spar deck bulwarks.
 
Nah, these are big sloops! A whole class of them (albeit individual designs). The only bigger non-razee sailing sloop was Constellation, which was built to heavy frigate dimensions. The 1840s class of large sloops were a good 20+ feet longer than the previous classes, which were almost 10 feet longer than the Wasp/Frolick/Peacock of the War of 1812. St Marys there is exactly 150 feet between perpendiculars. Jamestown, the biggest, was about 7 inches shorter than the rebuild Macedonian. They're all the size range of conventional 18-pounder frigates.
 
EDIT: Oh yeah, the other end of St Marys from that drydock.

That stern view of the St. Mary's is amazing, for all the details. I am surprised that she still mounted guns in 1874, and I'm even more amazed that they wouldn't have landed them prior to putting her into dry-dock!
 
Look how huge she is, compared to the man crouching on the scaffolding by the rudder! I always tended to think of the class as just "small sloops".

Talos, where are you finding these incredible images?

While I've seen many of these before, I've not seen them in this hi quality so definitely some nice finds

The first ones were Library of Congress. The recent ones I've been finding on the Naval History and Heritage Command website. http://www.history.navy.mil/A lot have been uploaded in the past year. All of these pictures I've been posting have been the small thumbnails, the full-size ones are incredible (you can really zoom in). I've been pouring over details on the Santee/Sabine, the various sloops, and steamers like the Hartford and Pensacola the last couple days. They even have one I knew of USS Dale in much higher resolution that makes me want to fix the spar deck I put on her, as well as work on the sloops to get them to this awesome late-19th century look.
 
Here's one of Cumberland in 1860, before the war started.
 

 
Here's one of Santee decked over. I think the aft window really shows the side pivot port for the Parrott.

 
And a great shot of Saratoga. Similar style stern to Portsmouth, but it has additional moldings. Harder to see in this thumbnail, you can zoom in and see it clearly in the fullsize TIFF on the website. In the Portsmouth picture above you can see the same mouldings on the tiny quarter galleries, but they don't extend across her transom. I can also tell in later photos of St Marys from the 1890s that she never got quarter galleries of her own.
 
 

The Naval Heritage website has a bunch of photos of Delaware's figurehead from your link, both the wooden one and the bronze replica they cast (as well as the physical casting of it!).
 
Continuing the discussion of the 1840s sloop sterns, here's St Marys, circa 1874. She's got the spar deck already, but no quarter galleries. Great look at the quarter ports though.
 

 
EDIT: An interesting detail I noticed on St Marys and Portsmouth both, the rudder chains wrap around the stern.

The Naval Heritage website has a bunch of photos of Delaware's figurehead from your link, both the wooden one and the bronze replica they cast (as well as the physical casting of it!).
 
Continuing the discussion of the 1840s sloop sterns, here's St Marys, circa 1874. She's got the spar deck already, but no quarter galleries. Great look at the quarter ports though.
 

 
EDIT: An interesting detail I noticed on St Marys and Portsmouth both, the rudder chains wrap around the stern.

The Naval Heritage website has a bunch of photos of Delaware's figurehead from your link, both the wooden one and the bronze replica they cast (as well as the physical casting of it!).
 
Continuing the discussion of the 1840s sloop sterns, here's St Marys, circa 1874. She's got the spar deck already, but no quarter galleries. Great look at the quarter ports though.
 

 
EDIT: An interesting detail I noticed on St Marys and Portsmouth both, the rudder chains wrap around the stern.

That really only held through the war years. Once peace reigned again, American ships actually became remarkable alike. Paint became standardized with the white stripe (then later extending the stripe over the bow), Any remaining gingerbread was either removed or painted black, with just a few decorative motifs like stars and eagles. You definitely see in the 1820s-1830s there was a standardization through the navy.
 
Probably from Constitution, as you'll notice Boston's is quite different. I think he really kept the same bulwark lines and just turned them into railings. Don't forget that the original six got extensive rebuilds, with different heads. Just compare the depiction of Constitution and President in my comparison, plus Constitution as she is now.
 
Speaking of Constitution and that picture you link, here's one of her old figureheads.

 
And the infamous Andrew Jackson figurehead that replaced Washington.

 
HMS Macedonian's. This was likely the only major piece of the original frigate to appear in the American rebuild.

 
And Ohio's Hercules figurehead displayed on the side of a highway in New York.

 
Some decoration details from Alabama/New Hampshire/Granite State.

Certainly changed. For instance, the berthed up bulwarks later in the war. I was just struck by how much it was shaped like those two (which were based on anachronistic drawings by probably a junior naval constructor as practice that featured those rail shapes, but as solid bulwarks). Obviously the whole image past the bow is even more simple than the rest.
 
 
Yeah, I know. The last Snow died a few years ago too. I was pointing out that the forum member physically lives next to the old site of the shipyard.
 
And now onto the new stuff. First off, this should raise interesting questions on Portsmouth's stern. It's definitely round, but the quarter galleries are faired in completely.

 
 
Portsmouth's gun deck after the spar deck was added on top (she was built open-topped). Three visible Dahlgrens, a Parrott in the back, and a sailor reading a newspaper. I have been finding some amazing Portsmouth pictures.
 

 
 
Being burned to recover her metal in Governer's Island, New York in 1915.
 

 
On the subject of the gunport windows last page. I think Constellation's constellation windows are even crazier. Also she looks amazingly sleek in her late-19th century guise.

Hmm, not bad. The railing on that one reminds me of the flow of New York and Philadelphia's draughts. Where they continue unbroken until getting to the bits abaft the cathead. You can see what I mean here in the comparison I did.

Looking at the plans, the Brandywine-class frigates have a very flat stern with rounded corners. Portsmouth is closer to Constellation. Looking at Germantown again though, with her quarter galleries on, her stern almost looks flat (despite not being so). Explains why she and Portsmouth only have two stern ports and no quarter ports too. The structure of the hull itself looks in between Brandywine and the Hartford style. You can see what I'm talking about with Germantown here.

I almost wonder if it got added around 1876 with the US Centenial that year. Lots of patriotic imagery. That's just a total guess on my part though. Notice they also added two smaller star badges on the back of the quarterdeck, which are blank in Skerrett's drawing.
 
@frolick
Her plans depict her without any quarter galleries at all. If they were added on later (such as when she got the spar deck), it might account for her stern being like that. Albany was the only one of that group to be built with a square stern.
 
EDIT: Just to muddle the waters more, her stern looks pretty round to me here.
 

Yeah, no kidding.
 
Unrelated, a nice shot of Portsmouth and Lancaster in ordinary in 1894, giving a good view of the sloop's stern. Big carving!
 

She was a training ship and not going to go into battle, probably moored for good by the time that was taken, so it's no surprise it's so heavily decorated and furnished.

That would be the I.L. Snow shipyard in Rockland, Maine where Sam Ladley here lives. Best place to start investigating.

Another interesting photo, this one from the US Navy Heritage and History Command, Sabine's salvaged billethead. http://www.history.navy.mil/our-collections/photography/numerical-list-of-images/nhhc-series/nh-series/USN-901000/USN-901319.html
 

 
And another one of Santee's gundeck, from further aft this time.

 
And Santee's captain's cabin.

 
Santee's gundeck looking aft.

I haven't found any yet. Of thse windows, I see three patterns in that picture. The first two on the left have three panes with vertical borders between them and the cannon still filling the port, then one with only two panes and one vertical border. The guns on the other three are removed, maybe turned sideways against the hull, and the window has an almost sunburst pattern to it. They appear to be half-ports too, but the ports on those three windows don't have the cutout on the lower half for the cannon barrel. I can probably draw up some sketches of them if need be. I imagine they were there for those two very reasons too. Those should be the officer cabins, specifically, so it makes sense they would be the ones fitted with windows. Look at the difference in diameter between the cannons on the gun deck and the spar deck. The above are likely 32-pounders, so the bottom are either 8" 63cwt guns or Dahlgrens.
 
You can also see the badge on the quarter gallery and the two half windows on it I was talking about too (the upper two panes on the center and aft windows on the quarter gallery itself)

Yeah, they can be really nice. Notice the forward pivot ports are open in the Sabine picture you show there. And yeah, could be very different. Different builders, different times, etc. They did have different bow designs anyway.
In reference back to the quarter gallery thread we had a while ago, I think I see two half-window plugs on the two aft-most windows on the quarter gallery, instead of just the one center one we normally see. Also the three-pane half-windows fitted in some of the gunports that still have cannons in them!
 
EDIT: Cropped the bit of stern out for a better view here. Tweaked the levels a bit too.

Huh, here's an interesting picture I found poking around LOC. This is Sabine and if you look up the full-size (88mb) TIFF, you can clearly see her aft Parrott sticking out of a slightly wider port out the side of the ship. You can also see the bracing for the forward pivot ports on either side of the (double!) cathead. Notice also the gunport covers hinged at the bottom on the stern windows, as well as the window plugs on the last few broadside gunports on the gun deck.
 

http://www.loc.gov/pictures/resource/cwpb.03820/

Like I said, it's mislabeled. Richmond had an open gun deck and even when a spar deck was fitted there were far fewer guns on her deck. You can see the name "Santee" on the sand buckets hanging on the left  as pointed out over on Civil War Talk.
 
http://civilwartalk.com/attachments/santee-jpg.39160/

Those are definitely mounts for the breeching ropes of the cannons. With having to clear the bench in front of the ports there, I wonder if they're more meant for the two-wheeled iron Marsilly-style carriages like you can see in this mislabeled gundeck photo of Santee. https://upload.wikimedia.org/wikipedia/commons/4/49/Gun_deck_USS_Richmond_LOC_4a14697v.jpeg(you can see the name Santee on the buckets). Those carriages make more sense for the stern anyway, since they allow you to shift around one or two guns easily without pivot rails or filling all four ports constantly. You can even store the guns normally out of there, so they don't take up space in officer country.
 
EDIT: You can find/download a bigger version of the Sabine gundeck (facing the bow, I might add) picture here. http://www.loc.gov/pictures/resource/det.4a14697/

It's certainly possible, of course, though I think I got them right, with the fourth main deck port being hidden by the curve of the stern. I point out the port by the bow of the stern boat being almost directly above, as well as the quarter gallery badge (probably a star) in the next space directly to the left, like on the modern stern of Constellation. http://l7.alamy.com/zooms/83f9395f348a497984c631fe8caedc81/uss-constellation-seen-from-the-stern-baltimore-d0w36x.jpg
 
It wouldn't make sense for the ports to function as quarter-gallery access. Not only would that be defeating a major reason for the stern shape (allowing fire over the rear quarters, eliminating a major blindspot  that men-of-war had,  but the quarter port is visible on all the plans of round-sterned ships with quarter galleries immediately forward of it. The Sabine plans actually shed some light there. They're missing quarter galleries and stern detail, so I overlaid them with the Brandywine's stern (which does have them) and they're an exact match. Rather than the quarter ports, they actually cover up the very last broadside gunport aft. You can see it in the heavy frigate comparison I threw together last year. 
 
You're right about Jamestown. The stern detail on the draught and on the real ship is amazing. Notice also in the picture that there are gunport covers fitted on the stern ports, hinged at the bottom and hanging horizontal. Her plan is so weird compared to the other super sloops of her construction program. Vertical, heavy stern, fitted with a spar deck from the start, and quite a bit larger than the rest.