Help identifying this vintage Omega

The reference number may be inside the case-back.

With so many variations during that era, it's almost impossible to tell a reference just by looking at it. I could guess it's a cal 280 family, but I can be way off. Pop the case back with a sharp knife (imagining it's a snap on back) and it will have the reference on the inside. Most probably something like XXXX-X. Then in the movement you'll find the serial number, a number with 8 digits that can help us tell what year it was produced.