'Phosphor Bronze', answers both questions I think
I must admit to always being very keen on the concept of the PB piston seal. As I understand it the reason it was not used in future Webleys was due to cost, rather than leather seals being an improvement.
IIRC the FWB65 uses a PB piston ring, and that was not a 'cheap', or half-arsed, pistol.
They are meant to have an extremely long shelf life, I seem to recall figures like a half million, or a million shot cycles bandied about, particularly with reference to the FWB65. For that reason I'd be inclined to keep it in situ...