Logun Mk2 silencer fitment O ring?

[CapitalBee]

A friend dropped by with a Logun Mk2 today as he was having problems with a loose silencer. Someone's had a brainwave when they thought of this design as the main centering component seems to be the O ring fitted on the adaptor of the silencer. The original seems a bit tired, so I was wondering what anyone has used as a replacement and with what success?

[Murphy]

I am sure they had a silver washer and somebody must have lost it and replaced it with a o ring.

But it was probably 20 years since I last handled one so that might not be 100%.