I bought it...Thanks for posting, M-theory, I owe you a beer! I'm lucky enough to be buying it for a friend/co-worker, so I didn't haggle over the seller's price, it seemed worth it and he seemed like he needed the $$ (also it didn't smell like it was stolen, hope my instincts are right). It IS a Mystery: very likely it IS all 531, but beyond that I'm lost. It came apart very easily and revealed 2 matching serial numbers on BB shell and back of steerer: 2318141, very small neat stampings. It's pretty clean and well built, but not quite up to Bruce Gordon levels, IMHO. I'm guessing American or British, and have posted those bad pics from the CR ad plus a detailed description on this Velospace site: http://velospace.org/node/1962
I will take better detail pics and post them to the same site ASAP...but meanwhile, any educated guesses??