Breech seals wanted for S2-1

Hi everyone. I am looking for some replacement breech seals for a Chinese pistol called a 'S2-1". It has been sold under many brand names, such as Westlake, Tech Force, Norinco, etc, etc. The seal in it is leather, but they leak like a sieve. I would like to replace them perhaps with a good quality O ring, suitable for airgun breech use. I could use a few, as we have a number of them at our local club that we just plink around with. I am attaching a picture of the pistol, if that helps. I believe they are 7mm X 2mm? but not sure as this is some info I came across on another airgun forum. Any help on locating some is appreciated. Cheers.

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