Chris- thanks you for your reply.
Unfortunately, I'm not entirely sure what I'm supposed to be fabricating, either. In the instructions, they call the cross piece a "trenailrail". I'm to use nails for "treenails". I suppose it is a belaying pin rail. Later in the build, they call for some braces for the fore topsail, and downhauls for the jib & flying jib, to be lead to this rail.
The other parts in the kit make sense, and I'm hesitant to cut up parts until I'm sure that I'm supposed to do so. What threw me off is they cut two slots in the part, and it is much larger than needed. As you noted, other builds don't have that rail, which confused me more.
Again, thanks for your help!
->Jack