The piston spacer according to Bruce (lots of detail on page 114/115/116) came in '64 for .22 models and a steel spacer with PTFE washer replaced the MK I type leather cup inner/outer. This was short lived before a thicker PTFE washer was utilised. Later again piston length was increased and a thinner washer used.
Again, Bruce states rubber breech washers came in with the D series circa Jan '66.