I'd think a Hatsan Mod 95 seal is the same.Pull the old seal and measure it. Harbor Freight probably has them if no where else.Just eyeballing them and they appear the same.
It's not exactly an o-ring. Look back as I edited my post.
The Mod seal from Hatsan will work.https://hatsanairgunsusa.com/product/break-barrel-breech-seal-5-pack/Or you stack o-rings and add floss if needed.
Nothing is ever easy......
I've used dental floss behind the breech seal at times. That works pretty good.