metridium once bubbled...
I wouldn't use a convertible valve because that lessens the likelihood of some yahoo picking up one of my tanks by mistake and possibly taking it past its MOD, stickers notwithstanding. 300 bar regs still aren't that common among the general populace.
Ok, first of all, you should have your name on your cylinders, and the mod of the gas, along with Nitrox stickers. Secondly, 300 bar regs are very common. Where are you diving???