I can confirm I never had any trouble getting 540 threaded bottles tested, and in the very unlikely case of being able to find anybody who can fill them (scuba shops are close to useless for this, I have found) one is far more likely to be able to fill a 540 bottle than anything else.
I do also carry a 540 to UK bullnose BOC bottle adapter but it doesn't get used, except when filling from a big BOC bottle and I got a hose made up for that anyway. DIY refilling is the only practical way for regular flyers.
The MH kit is very good for oxygen usage but has two drawbacks: when the batteries go flat, it fails SHUT (no oxygen), and the short but intense oxygen pulses are not as comfortable up the nose as the other systems.
Having used the Precise Flow mechanical demand reg kit for a while, I would probably buy the MH kit now, on balance, but would get it wired to the aircraft electrics (as well as having the batteries inside) and would get
two of the demand regulators for redundancy.
The bottles I got from Aerox are U.S. Catalina cylinders which are either scuba or medical types, resold into the aviation market. They are rated to 200 bar but there is no way to fill a number of them to 200 bar when the BOC source bottle holds the stuff at 200 bar
