Guessing it's an older, leather seal one?

Many would say to stick with the leather one to keep the more "classic" character.

Although Mick, T20's, idea of fitting a complete later piston with synthetic seal very much makes sense.

Some will also advise that, once you've converted a leather one to synthetic, they're nicer if you sleeve the transfer port down slightly.

Of the synthetic ones that I have personal experience of, the factory one and V-Mach ones are both excellent. Many do speak very highly of the Australian ones that are available, but I have no experience personally.

My old wardog '35's leather seal was shot. I ordered a new leather one from Knibbs (talking 9 years ago here) and a synthetic one with adaptor arrived - complete with larger than expected credit card receipt! But it does shoot very, very sweetly. Only doing about 7ft.lbs, so maybe a leaky breech. But as I have loads of others and only shoot this at short range with open sights, I might leave alone. The ultra-sweet cycle maybe a result of the "void effect".

Back in the early 80s I converted one to TR Robb PTFE seal. Very quick action, very accurate but the surge was horrific and the trigger finger didn't half sting after prolonged sessions!