The 540 thread I know well - that is what the US cylinders have. But this is big - about 20mm dia. The thread I have seen is about half that size.
Are you saying there is more than one "standard" and that when a King Air pilot lands somewhere and needs his emergency o2 topped off, somebody needs to mess around with adaptors? This may be so, in which case there is no simple solution.
That URL is dead, currently.
This one might be relevant, perhaps, but I cannot see enough detail.