Hello ronabarr, and greetings to the GTA.
Calmark gave good info.
I would like to add though....please stop shooting the gun NOW! The old seals on Walthers were often, even in the old days made from Silicon and these just disintegrate quickly once the power drops. This will FOR SURE cause the piston to hit home metal to metal with increasing damage each and every shot! The assembly has the threaded part of the seal (the bolt, like a stud) attached to the piston, unlike many other German guns, and this will also bend and the threads will be ruined for future seal replacement. Re-threading these is tedious as the size is even here in Germany non-conform. This I know from personal experience.
I am sure you will eventually find a new seal, maybe even the existing one is of leather, and this may even be able to be fixed by someone easily. Don't fret, but be prudent and in due time the wonderful gun will be shooting again.
PS, never sell it, it is quite valuable, and not only for your personal reasons.
Greetings from Germany,
PS: The pic shows a kaputt seal. Kaputt is German for fubar.