it might clean up... the thread is something weird and BA IIRC. Not the usual schraeder type 6VD or whatever they call it.
the photos are confusing me a bit - why does it have o-rings outside on the tail of the piston ? Do you have the "dummy piston" ?
Even on my early countryman, the "spring guide" / dummy piston is one piece with the end cap, and just slides into the piston body. the piston has a couple of internal o-rings and a backer fitted to seal it, simples.
for yours to work, as you say, the dummy would need to fit inside the recess in the end block and seal... and stay put whilst being fired, which seems unlikely.
So I'm a bit confused.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.