Just in case anyone is interested, the newer model Diana Air rifles (T06) also use O-ring breach seals. Not sure if any of the other earlier models also do though. Quite a few of the T06 rifles use the same size O-ring which is a 8mmx2,5mm. I've found that the 90 durometer are a little harder than the 70(standard) and last a lot longer, but any variant from 70-90 durometer will work fine. I've also replaced the breach seal on my LGV with the 90 durometer variant, but it's size is a 7x3mm.
Anyway, just some odd info I thought might be useful to some.