Omega 18k Gold 1920! I think is fake (help?!)

Hey Omega lovers,

I ended up inheriting some stuff from my grandmother, and this time pice is one of these.

I winded it, and the movement work just fine. The dial says Constellation, and on the back 18k 1920. Reason why I think this is fake. Constellation wasn't even produced in 1920 or I'm I missing something? Like, constellation movement in some old omega case?

Only you guys can help me here thanks

Your suspicions are correct.

As a rule for Omega watches..they never stamp the 18k logo on the outside of the case back. So this one is fake and I assume not gold..