DuoFast Coil nailer Model KDN-50A Main Seal.

Bostitch N16 Framing Nailer, rear main seal and the piston rear end stop.

The Seals for these old air coil guns are impossible to find and I don't think they even make them anymore. I printed mine in TPU, they work fine and so far they haven't failed. If they do eventually, just print more Seals!
TPU filament gives that bit of flex to stretch the seals into place or absorb shock. Printing with a infill might even add to shock absorbing ability.