I believe that is the case makers mark, not a hallmark.
My favorite omega stamp. It's Omega French market stamp.
Solid gold case should have 14k or 18k stamp on the case back. No stamp - mean that the case is not solid gold.
Usually gold plated cases has stamp between the lugs.
Does yours have any stamp between lugs?
If I'm not wrong, the old Omega cases have a gold plate up to 80 microns.
It takes a lot of effort to remove 40-80 microns gold.
Your case back is stainless steel, it appears to me. Omega has never (in my opinion) gold plated steel case components. Gold SHELL? Yes! But never gold shell on a stainless steel case BACK! If the back side of the lugs on your case are steel in colour, with the gold tops on the lugs tells me they are going to be gold SHELL, not gold plated. The bezel should be karat gold.
Your case back is stainless steel, it appears to me. Omega has never (in my opinion) gold plated steel case components. Gold SHELL? Yes! But never gold shell on a stainless steel case BACK! If the back side of the lugs on your case are steel in colour, with the gold tops on the lugs tells me they are going to be gold SHELL, not gold plated. The bezel should be karat gold.
I’m Look at the picture at my phone, the case images are to small, can't define well if it's steel color or gold.
I’m Look at the picture at my phone, the case images are to small, can't define well if it's steel color or gold.
There is a French gold hallmark in the middle of the medallion