Taking the cable stops off the 828 and putting them on the proto is an option ... IF they are the same width. Those stops aren't flat at the bottom. The sides follow the curve of the main tube's rounded corners.
Also, there's the question if this would indeed be the way to go. Aluminium Sbikes had removable cable stops, but the steel ones had the classical ones that were welded on to the frame. This looks like a steel frame to me, so it would make sense to get them welded on. Then again the quality of the welding on this frame makes me think that it might be a Verlicchi-built frame, and so were all the aluminium ones. So on that basis I could use removable stops.
Even small choices like these are really difficult.
As for the swingarm, swapping that is not an option. They are too different in style, and perhaps even in width.
There's another problem in that area too, as the rear brake's cable stop is normally integrated in that bar across the front of the swingarm. The prototype frame doesn't have such a bar, so I'll have to figure out how to get canti brakes to work. Or weld on the supports for a similar cable stop like the 828's swingarm.
I can't use V-brakes because those were never on any Sbike. Not sure if there were Maguras in 1991 either, and the suspension would be in the way of Magura boosters as well.
Looking at the pictures of the 828 and the proto, I'm now starting to think that it could be a genuine Sbike. Either that, or someone had a full-suspension Sbike for comparison and one hell of a machine shop.
Look at those folds along the length of those support struts. I don't see an amateur doing that.
I may have a way to contact Sbike's former CEO (gotta love Google), so when I get the frame and have some pictures, I'll try to get in touch with him and hope to get confirmation.
However that won't solve everything.
How do you build a bike to the correct factory specifications if the bike/model never existed in the first place?
Should this one have Campagnolo or Shimano parts? STX-RC? LX? XT? XTR M900? Did those even exist in 1991?
Do I use a Velo VL-190 saddle (like the steel Sbikes used) or a Sella Italia Alpine/Turbomatic/Flite-Ti (like the production FS models)?
This is going to be one hell of a build. Every single component has to be thought through and I need to be able to justify my choice as well.
I'm pretty sure that my other projects are going to be put on hold until this one is finished.