A lot of Gems never had seals they relied on a good fitted piston and the use of oil, I have come across many that have a leather washer in the bottom of the cylinder to minimise metallic contact when fired, the fact that the piston was hardened probably meant that no seal was ever fitted.
Instead of fitting a seal to the front I've found that grinding a groove behind the head and fitting an "O" ring
Improves the usability somewhat.