It looks as though a leather seal is first put on, then that washer and then the spit inner section is spread open wedging the washer onto the seal. Without heating up the piston red hot and somehow narrowing that inner section I can't see (from the photos) how you could remove the washer without damaging it. I guess this is another design that was not intended to be repairable. I find that quite odd considering that this is a Frank Clarke design. Very crude.