Help identifying gold Omega dress watch around 1990

Hello we inherited this solid gold men's dress watch from my father in law. He bought it from an upmarket jeweller in London around 1990-91. I believe it cost around $1500 to $2000 at the time (AUD). Have tried to identify the model but serial number not computing and no name as such on the front. face is white, it just looks slightly yellow in the photo. Band is a recent add on, original band was also leather.
Hoping someone can assist

Hi
The mark on the back is hard to work out - looks like a makers mark
Inside back case has a few numbers - 18k 750 (as in gold I assume) also the longer number 196750

Thanks for that. It’s a nice watch to wear so we are hanging on to it