This experience may be of use to someone?
I initially replaced the decayed leather seal on my Mk3 335 with a PTFE seal but without much success, and thought I could feel air leakage on firing coming from the cocking lever slot. Second attempt with a 28mm blue Diana seal and adaptor (from Pr*t*k Supplies) was much better. I don't think I have the infamous 335 cylinder bulge close to the manufacturers stamping from external measurement. I know some have had success with PTFE seals in 335s, but it didn't work for mine.
Now very consistent and accurate: a favourite for using with the excellent open sights.
Best Regards,
Bill.