Missing numbers

[allegedly]

Hi all, Rob, new member here. Very good site peeps.
I have a question for you. Under what circumstances would the Ident numbers be missing from between the strap lugs on the case?
I am looking to buy an older rolex and one I have been checking out appears to have the numbers removed. The metal is fairly roughly scored, by watch finish standards.
It raises a few questions:
The obvious one, is it a fake?
It is well aged, obvious signs of wear/age. The face is a bit dull, dirty. The fingers are slightly, almost microscopically pitted with age, tarnishing. The crystal (plastic) is slightly dulled with scratches.
So, to my untrained eye, it appears to be in the condition I'd expect a fifty year old watch to be in. If not better, looking at my Seadweller which I've had from new for twenty three years!
I've been told that if a movement had been changed to a different, better case, for repair purposes presumably, the numbers on the case may be erased. Is this possibly the case? (no pun intended).Thanks in advance, Rob