Is or was the tank chromed in part or whole? If so, are there traces on the threads? Might need a die (or a nut with a slot cut across one flat) running up the threads?
If the threads are fine / fine-ish, than it can only really be BSC at 26tpi (as it's meant to be), possibly BSF at 22tpi, or UNF at 24tpi, outside chance M8 x 1.25 or 1mm. They're too slim for 9mm. BSW or UNC would have been a bad choice for them, for being too coarse with too thin a minor diameter, so I doubt it is either of them . . . but you never know!
I'll stick my **** on the block and say I think that, because the design of these things has been pretty much standardised now among the various tin-bashers out there, it'll be BSC in need of a clean-up, but hey, could be wrong of course!