What a beautiful and interesting bike! To confirm what is fairly obvious from your pictures: is the brake bridge designed for a recessed nut? which side of the bridge has the larger dia. for the nut? This should help to resolve which side of the bridge/stays the caliper was meant to be mounted. The Colnago in the Post #22 link has 'reverse' mounting built into the brake bridge (recessed nut facing outside the rear triangle, caliper toward the seat tube), and everything here looks very much to me like 'standard' design (recessed nut facing seat tube, caliper outside the rear triangle), but you would certainly know better than I - and that's not to say that a workaround couldn't be implemented to mount the rear caliper in 'reverse', but why design a workaround into the bike when it would be simpler to reverse the bridge when building the frame? ...and why design the cable to exit from the rear of the TT on the drive side for use with a standard caliper in the standard position (cable to non-drive side)? A bit of a mystery. Perhaps this is just a matter of making do with what was on hand for a bike that may have never been intended to make it into public hands (though I'd expect rather plain paint in that case...). The idea of the bike being designed for Shimano hardware is just a little hard to stomach for me personally, but it certainly isn't without precedent. Gary's is a good theory that certainly makes sense (and he's certainly more knowledgeable than I am!).