Paul O'Donnell made some excellent BSA piston washers a few years ago. I bought a few at the time and they are better than any others I have bought from other sources in the past. Paul usually sells parts for the Crosman CO2 pistols and of course his excellent repro Webley Service sights. Quentin Chobham may have some if he's there too.
Kind regards,
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.