I can't remember the sizes of the O rings, sorry.

The exploded parts in the photobucket picture are from a transitional mk1/mk2 gun, probably manufactured around the end of 2004. Genuine Hull Cartridge import, 12ftlb.
It was one of the first .177cal and one of the first with the sporter stock and the better screw-on silencer, but lacked the reinforced cylinder thread which was standard in the true mk2 a few months later.