Cheers Lakey...
Mine is older, two hole trigger, 16,xxxx serial. defo just has a single screw hole in the face of the piston, but what's on there right now is defo non original, as I suspected. No big deal as it's quite well done, but the leather washer is just toast.
On mine what would be the inner washer + plus metal dish, is in fact a single solid metal bush, with countersink in the centre for the retaining screw. In principal it would be fine, but I think the OD of the bush is too large, leaving the "lip" of the leather outer seal too thin, hence it's degredation.
Thanks again, clarifies things nicely...
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.