Hi Brad,
The Model 12 Smithsonian (or "Smitty" or "Heavy") was introduced in 1953. I put yours somewhat North of '55 primarily because the keeper on the sheath is diagonal and not horizontal. The blade looks either chromed (Which the shop would do on request) or polished after it left the shop, but I can't be sure from your pics. The sheath was likely dyed black by an owner. The stone is not original. Should be a Norton. All said and done I think you paid a fair price. Shel may be able to elaborate further.
Best,