Serial and reference are engraved on the lip in between opposite lugs. They are small and if the case has been polished or on a bracelet easily worn away.
I had a 1153S where I could only just read what was left of the serial at a certain angle, in the right light, with a strong loupe.....

: Where do I find the serial number on my 1153N?

: http://www.onthedash.com/chronograph/carrera-ref-1153-n-first/

: It says edge of case but I can't find it?

: Here is my watch

: https://omegaforums.net/threads/heuer-carrera-1153n.36701/

: One of my favourites to wear.