Umarex fill nozzle identification?

Hi folks,
My first post here as a guy transitioning to air propulsion from powder. A result of being under the influence of YouTube videos showing the tremendously popular undertaking of night time, air gun rat shooting in europe. I had no idea that such a thing, was, well, a thing!

Under that spell, I bought myself an Umarex Gauntlet G1 in 25 cal and installed a $1,300 thermal scope to experience my own thrill in this. With the right pellets, I have no problem putting five rounds inside of a dime at 50 yards. I've shot too many field mice to count now at night out as far as 55 yards off of a set of bipod version, Primos Trigger Sticks.

Following the learning curve of thousands and thousands before me, I've come to realize that hand pumps have to have precarious maintenance to avoid driving moisture into the workings of the rifle. I've finally come to the point where my air fill port is leaking because it has corroded enough from resident moisture I didn't know it was accumulating there. Not hard to associate, but this revelation occurred after spending a week on the Atlantic coast shooting mice... i.e. high humidity tainted with corrosive salts.

I've tried to find a fill Port replacement that meets the dimensions of the one I have that came with the rifle. It is 0.370" OD, or 3/8" diameter and looks to be a standard 3/8 TPI pattern, not a UNF or NPT or somesuch. I'm not home right now to put a thread pitch gauge on it, otherwise I would provide that.

8 mm is too small, 10 mm and 3/8 NPT are too big. Can anyone inform me what thread pattern this is and where I might find a fill port that has it?

My thanks for any help the forum Community is willing to offer.

-w