Could be 8mm? Can you remove it to measure? I had a leak in the same area on a Merlyn and it was 8mm. My notes from the time are:
'Leak found at the pressure gauge brass adaptor to the cylinder. Seal is 8mm Dowty. Bought from (id 8mm, od 14mm), fitted and hopefully leak cured. Take care if doing it again as the brass adaptor screws into a ‘loose’ part in the cylinder, so keep cylinder horizontal, gauge up, when removing gauge.' I have vague recollections that the seal was a 'German' size.

Now I cannot vouch for the accuracy of the above for your rifle but I remember being surprised that the Dowty was not what we might call a standard size.

Cheers, Phil