Hi, i sorry for signing up and going straight to pick your brains, but I am a frequent member of other watch forums and reckoned that this would be the most appropriate place to discuss what appears to be a mystery to me.

I recently bought an Omega Seamaster Cosmic from 1970 and from the dial I identified it as reference 166.026, identical to this one.
Now, on the back of the case, the reference number says 166.036, which would point to a later model with cal. 752, so I was assuming that the case had been swapped at some point. All characteristics of the movement pointed to 565, but the watch was stopping randomly, so I took it apart and the movement was indeed a 565 (and that a screw had come lose, blocking the balance and/or rotor).

Now to my dilemma: The case back says misleadingly ref. 166.036, while inside the case, the right reference number is stamped in, ref. 166.026. Please see the attached pictures.

Is this common, or how would you explain it?

Thank you!

 

The 2 ref # (166.026 and 166.036) were for the Seamaster Cosmic line. The .026 was a cal. 565 and the .036 was for a cal 752). In a number of instances Omega cross used cases and case backs for different models. Typically these were marked with both ref numbers inside the case. In this particular case, it is quite possible that the case ref number on the inside was marked during production and the outside ref number was added at a later time point. There is a good picture showing the 2 refs side by side in AJTT page 291.

 

Thank you for your comment Joe. I know that both 166.036 and 166.026 were produced side by side, used identical cases and the only visual difference from outside is the day/date on the 036, but I have to assume that in my case, the right reference number inside must have been added at a later point, while the case back was already marked before assembly. I couldn't explain why Omega would mark the case with the wrong number after the caliber and dial were installed.