I have a gen. Seamaster. It is called a helium valve. You can research its use using Google. In short, unless you are Jacques Cousteau you don't care what it does.
It screws-unescrews on the gen. I have never touched mine on the many, many, many years I have had it
I have no clue how it is on the rep. I can tell you that judging from pictures the reps are spot on. Biggest giveway for me is the lume dots in the dial, but maybe because mine is old.
Regards,