It would be a start if someone could tell us how the BSF piston washer is fitted! I can't remember for the life of me, but then I have probably not had a BSF apart since the late 1970s.

If the seal is held by a screw then it should be easy to get a PTFE head turned and attach it in the same way. If the seal is held by a rivet like the Original 45, I'd turn the rivet section off and drill/tap a central hole (actually, I'd have to get someone to do it for me because I don't have access to a lathe nowadays ).

I'm not at all convinced by the notion of turning the head of the piston down to enable a clip-on seal to be fitted.