Hi Ian.. These 21 strands ( cable-laid ) were pulling your leg, weren´t they? In fact I can´t tell their exact number, but for break resistance and to come up with a 70 mm diameter it could have been made to these dimensions.by her master designer Georg W. Claussen of Tecklenborg/Geestemünde.
Fabricate some rollers , if you feel like it, but remember their tiny diameters, and wether it will catch at all some eagle-eyed onlookers´ glance at the model : ca. 450 mm : 150 = 3 mm dia. And yes: the steering-cable runs through openings at the foot of the poop-bulkhead right to the after steering quadrant, not up to the poop deck itself..
I am the happy owner of the THE BOOK : "Königin der See - Fünfmastvollschiff Preussen" by Horst Hamecher. Out of trade long enough, but you might find a copy at an online-marketplace, like this one :
https://www.ebay-kleinanzeigen.de/s-königin-der-see/k0
Good luck
Marten