I got my first Omega yesterday. It's a 1973 model 166.0205 Seamaster with the 1012 movement. It is in fantastic condition with a real Omega strap and the original signed crystal. I paid $400 for it and think it's worth it.
I have a similar question as Jonathan was, I got an Omega Costellation 21 rubins from my uncle. The hallmarks is on the outside of the backside of the watch (18k 0,750) and the year 1943. The Omega mark is in the strap as well. I am curious to find out some information.