It seems to be mostly Diana and Langenhan types of Gem pattern that may have a piston washer of some type fitted, have seen the usual cup washer types as fitted to the later Diana 6 as well as the more unusual stacked leather disc type and the greased hemp string type as well in some LZ/FLZ Gems too.

EG and Marck Gems are usually a machined fit metal to metal and they work surprising well if not too tired and worn...

If you have a lathe its quite easy to cobble something up, but if not and the seal is knackered then you have to improvise as best you can...if you want authentic then cut off the old crap off the piston, punch out some oversized leather discs, punch centre out and put a few slits in the leather radiating out from centre hole, soak it in warm water for 10 minutes or so and see of you can stretch it over the piston head, if successful repeat...once you have all the stacked discs on the piston dip it in boiling water for 30 seconds to a minute to shrink the leather back on the piston head leave it to dry and shape to final size...