As noted in an earlier post, my new DISA O-ring is 50 mm I.D. and 3.2 mm thick.
I replaced the old one a couple days ago and it measures exactly the same.
I noticed when removing and re-installing the DISA unit, there was resistance both ways, presumably because the O-ring is meant to be a tight fit.
Those red ones look quite thin. Be sure to measure your old O-ring once you get it out.
My old one still seems usable by the way; not hardened or cracked/shrunken at all.