I don't think that's a Mares. It looks more like a ScubaPro Mk2 clone. What makes you suspect it is?
It was connected to Mares 2nd stages and has a Mares Yoke clamp. But that doesn't mean much though, it's easily changeable.
The design is defenetly piston unbalanced so mk2 style indeed. And it's probably from 90s to early 2000s.
Still no luck identifying it, could find anything in googlefu as well