There may not be one other than "50", but I agree the "50" probably IS the size. If there's a plastic cable guide under the BB shell, it *could* be under that, or on one of the dropouts, or on the seatlug...nothing on any of those sites? Maybe on the steerer (but you have to take the fork out to check there). Some of these small builders never used a numbering system. I think it's earlier than '85, since it's spaced for a 5-spd. rear. The 600 stuff ain't really a downgrade, and the RGC brakes are probably nicer than what it *may* have come with. I assume the hubs are Campy Gran Sports (no oil hole clip), and with the Weinmann rims these are the "weak spot" in this bike's gruppo. Not THAT weak, but if you need to talk the seller down over something, use this. And if it's built with something "less" than Columbus SL, more ammo...drive a hard bargain and good luck!!