Sorry should have been more specific as Bod has said its the oring that sits between the reg and the neck of the bottle, so not inside the reg itself. Also the reg isnt a branded one, i don't have it with me as its a mates bottle.Other than the one that seals the regulator on the bottle which isn't inside the reg.
Seeing as regulators and bottles seem to be pretty much universally threaded the same it doesn't really matter what brand the reg is they all take the same size o ring .Sorry should have been more specific as Bod has said its the oring that sits between the reg and the neck of the bottle, so not inside the reg itself. Also the reg isnt a branded one, i don't have it with me as its a mates bottle.
Cheers Guys
Ben