Congrats on the nice heir loom!
There are some variances based on outside appearance, but it is most likely reference 165.020 with calibre 552 movement, that was made in the mid 60s.
This is a delicate watch and not easy to open by someone inexperienced on this particular watch. It's not really important to open it risking damages just for some info. It's better to have it serviced by some experienced watchmaker and get the pertinent info at the same time.
http://naligazone.com/index.php/data-watch/omega/caliber-5/481Click to expand...