I just remembered strange information I saw on another Enfield forum written by "Ric" in 2014:
"There are four pistons available for the ElectraX, sized A,B,C & D.
The top of the barrel should be marked with one of these letters to enable the correct piston to be fitted during initial assembly. A is the smallest and D the largest. The D piston is almost an interference fit in an 'A' bore, so an undamaged but not too heavily worn 'A' sized bore can be honed to an 'as new' fit on a D piston. "
Has anyone heard about this?
The original piston of my 2005 Electra indeed bore a letter "B" and new replacement from our host's a letter "A".

