Trying to find information about my omega watch. Anyone can help ?

Hello, I'm new to omega watches.
A few days ago I found an old watch that belonged to my grandfather. Not with the box or the letter.
and now I'm curious about the model, and I want to know if it's original or not, because when searching on the internet I see that all omega watches have a serial number on the back, but not this one.
is there someone who can help me?