Without wishing to sound nasty please please please disregard any video you find which states that Original or Diana Model 75's had a leather piston washer as uninformed bull****.
All Mod 75's had synthetic rubber seals, most original factory fitted seals will have perished and hardened into a crystalline sugary toffee consistency by now, along with the breech seal and the buffer seal. More modern replacement piston washers, buffer and breech seals will last longer but again, many rifles will be on or due for the second replacement seal set by now.
Those "gunsmiths" on youtube who claim that the original seals were leather don't know their ars*s from their elbows! Be very cautious of what "advice" you follow! Some of those strip down videos are extremely misleading and involve dangerous practices!