Looking at the pics, I assumed those canti mounts were an aftermarket improvisation, but looking at other Mountaineers online it seems like they really did sell them with clamped on posts, apparently to be more versatile with respect to wheel size and brake type and location. On the seat stays especially, I would think it would be hard to clamp tightly enough to keep them from rotating under load. Maybe that's why the shims are there.
Anyway, I would think that such an early example of Moots work would command a pretty high price, but I don't know enough to put an actual number on it.