Omega 2374/4, problem with loose snap back

The case back of my Omega 2374/4 (mid 1940, picture attached) doesn't snap in position and will just fall off. Inner dia of the case is 31,55 mm and the outer dia of the "snap edge" of the caseback is 31,45 mm i.e. 0,1 mm too small.
I don't know if this is due to wear or if there shall be a special seal/snap ring in the grove?
One of the former owners had glued the caseback in place which I doubt is the right method.

I would appreciate very much any advise or suggestion to solve this issue.

Ok, thanks a lot!
Now the challange will be to find one. Any suggestion where to find it?

Great, thanks!