OK. Just so I'm understanding this correctly..................................
The fitting
on the regulator is MALE and it is CGA-346, correct?
The fitting
on the H2 tank is FEMALE and it is CGA-580, correct?
Obviously, the fitting you have now is MALE CGA-580 on one end
and MALE CGA-346 on the other end.
I've scoured the Internet for the past 2+ hours looking for an adapter and have come up with absolutely nothing.
The best I could come up with is using this CGA346 fitting to attach to your regulator. This CGA580 fitting to attach to your tank. And this 1/4 inch coupler to join the two 1/4" nipples from the other two fittings.
I don't think they make a simple adapter to meet your needs. I looked everywhere. You may be able to find these same fittings cheaper if you look around. I just linked the first ones I found. I'm not sure what else you can do with the setup you have. What I think the real issue is is that oxygen masks are not meant to be couple together with nitrogen tanks - AND you have an O2 regulator, NOT a nitrogen regulator. I think that is complicating the entire thing. Maybe you'll find something better by looking around more. Let me know what you come up with.