I assumed it was the regular bottle thread M18X1.5 mm in the bottle not the fitting screwing into the rifle.
But i was not able to unscrew mine, even with what i feel was ample heat was i able to unscrew the AGT fitting in the bottle.
So i changed to just use my 480cc Maverick bottle, and then put a stock FX part that the bottle can screw into.
Now this still mean i loose all the air in the big plenum pipe when i reach for the regulator, but that is still a lot less than what is in there + everything in the bottle.
The part i put on the gun was just the standard FX bottle connector. M12X1 mm on both sides.
On my request Huma made one too, which i think are valved so you do not loose your air in the plenum pipe either, well at least if you just swap a bottle.
FX Airguns Bottle Adaptor For AGN Vulcan III by Huma-Air
www.huma-air.com
Huma said they would send me one to test when they had them made, but never did

so i cant say for sure on this.
I am OK with the standard FX part as i now have my own big compressor so air are no longer a super valuable ressource for me.