Lots of pistols have an air passage like that. The overlord I have has it, but instead of the crappy proprietary urethane seal the ptx has, the overlord has a big piece of rubber the same shape as the cover that will probably never go bad.
The point of the air transfer thing is so the bolt can be farther back on the marker, leaving more barreland longer feed tube while still being flush at the business end.
The USP goes straight from the valve to the bolt. I can't seem to recall another pistol like it so the USP may be the only stbb pistol that doesn't have an air transfers thing on that side.
*Edit I didn't know there was a non-cartridge valve. The ones I had were all cartridge made of aluminum. I've seen both brass and aluminum cartridges across the internet but didn't know there was a standard one. How does your valve mount in? The cartridge ones just rested against a cross pin and the npt bolt was kinda in the middle of it. If you had the npt bolt out you could shoot out the valve by dropping the hammer.
Last edited by smiffington; 08-24-2018 at 05:01 PM.