Elmo I think you mean 10mm I.D :)
My vent plugs are a smidge over 10, so I recon probably 3/8 is closer.......... but 10mm won't fall off.
Anyway here is my chance, its another thing that sh!ts me about Bunnings. You can't buy a 10mm o-ring anymore. noooooo you have to buy a mixed pack of about 100 all the way up to 50mm, and 4 of them fit. Kinda.