This narrows it down to either oxygen, fuel, or sheilding gas for
welding, or CO2 for beverage carbonation.. Knew it wasn't Oxygen - as
O2 bottles have LH threads.
the invalid, and use a net.

Do oxygen bottles have LH threads there?
Here ( Australia) they have a RH thread but acetylene and
LPG/propane?butane gas bottles have the LH thread. The nut part of a union
for these gases has a groove turned in the outside of it to show that it is
a LH thread.

Tom