I will leave with the experts... If we were talking about speed master or sea master I could have helped...
Fake. And an awful one too. There was never an Omega Costelletion it was a Constellation. Fake hallmarks too, no gold content in that one.
A search of "fake" would have led you to this thread: https://omegaforums.net/threads/dont-fall-for-a-fake-vintage-omega.6235/ Everything you need to know about that watch would have quickly been answered - especially the hallmarks on the OUTSIDE of the case.
Lol. Wow. Pretty neat that Granny collected fake watches in her later years. You don't see that every day. Uncles, I get. Everyone knows uncles love fake watches, but fake-loving grandmoms are pretty rare.
Methinks grandmom has been on holiday to either Thailand or Turkey. I would be wondering if it's a fake uncle dressed up in your grandmoms clothes, maybe something like this.
If you can't get the dual to come off the movement it will almost certainly be Formica as it it fitted with contact adhesive. If it comes off and there is evidence of burlap or canvas backing it will be Linoleum.
By the way (saves me from starting another thread), does this look like a redial? I mean, it says T Swiss T, but I don't see no lume dots...
On the one hand it looks like a hand painted cloisenne dial. On the other hand.... the hands are too short.