. . . the ref number is stamped in the caseback, but even without that photo it's easy to tell by the caliber number. Ref 2913 employed cal 501 from its inception thru its last iteration; when cal 550 / 552 replaced the first generation full-rotor Omega automatics the reference number changed to 14755, even though the outward appearance remained similar to ref 2913.
The example Dirk found is original overall, although I'd like to see a head-on image of the crown, which from the images shown so far might be a replacement. No bezel? No problem, says me! I'd still take it gladly.
Art