"Steven P. McNicoll" wrote in message
In a previous life I was in aircraft maintenance. Aviator's breathing
oxygen systems were serviced from just two sources; compressed oxygen
bottles or liquid oxygen carts. The only other compressed gas bottle used
for servicing aircraft was nitrogen. I don't recall what measures were
taken to prevent servicing oxygen systems with nitrogen, other than colors
and markings on the bottles, but servicing oxygen systems with nitrogen
would be a very bad thing.
Different fittings.

moo