I am wondering if this Constellation is original? Looks like a redial and there is neither the usual .750 logo nor reference no. on case back.

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Likely French

 

I'm not familiar with the case-back, but I will say that if it's a re-dial, it is very well done.

 

https://omegaforums.net/threads/18k-omega-constellation-case-back-question.64854/

 

French constellation case, but the other french connies Ive seen have FAB SUISSE on the dial.

 

Has an eagle hallmark stamped on the side of the case

 

Possibly correct for the age of this watch.
Not sure when the cut off is but there are lots of 60s French cased Connies without fab Suisse on the dial.
@cristos71 is likely to know.

 

In my experience the 3xx bumper chronometer's have Fab. Suisse on the dial and the later 5xx series don't, although I would not claim this to be the rule, more my observation.