They DO fail on a 300 bar cylinder. I blew 3!!! Apeks replaced them each time and insisted that they should withstand 300 bar pressure but they don't. However cautiously I opened the valve, they always blew, not always the first time but always within the first 4 or 5 times. You can get a neat spg (Beaver) on a 6" hose from Divers warehouse for not much more than a button spg costs, by the way.
I have one on one of my stage tanks. Its OK for telling you, you have a fill bit I wouldn’t be looking for accurate gas pressures from it I use a normal gauge for accurate pressure tests and then use the button to tell me its still full. I like them because they get rid of a hose and my 232 bar one hasn’t had a problem and I have had it about 18month (180) apx dives.