70s or 80s Omega Identification

The reference number is 166.0120, plenty to google off of that.. Gold plated case, aftermarket 10k gold-filled (like plated, but a thicker) bracelet. Not much else to say, Omega 1020 movement, serial number places it to ~1972.

Writing on the inside of the case back is a watchmaker's mark. A watchmaker wrote down that he serivced the watch in the middle of 1983.

Value: You can search by the reference number, but: Gold Plate is worth less than stainless/etc, there are quite a few in similar condition for ~$600, which is perhaps the higher end for your watch.

Last 3 sold on ebay were for:
$450, but claimed to be recently serviced (which would cost ~$300). It DID have an original bracelet, though not one in good shape.Case was similar shape to yours.

$600, but basically near mint.

$345: A bit rougher of a dial, but a stainless case.

By my read, yours is $300-$500 as-is at most.

Sentimental value is going to be a majority of the value though, the 1020 movements aren't particularly nice/collectable movements, and the 70s were rough on the mechanical watch industry. IF you want to wear it long-term, it likely needs a service (~$300 to have done).

Oooh neat, TIL! Thats a very interesting, if marginally less-useful setting mechanism for the day.

IMO, a nice light-brown/light-tan suede is going to look best on that (but I'm also addicted to gold + tan suede).

I'd VERY much suggest finding a good watchmaker in your area to give it a clean/service, particularly one who can replace gaskets to make sure it is water tight. It would stink to lose your grandfather's watch because of a handwashing incident. IF it hasn't been touched since 1983, it is very much due!