This is a Mountain High CGA-540 (axial-out, XCR, single outlet) regulator would be used for an oxygen bottle that has no gauge on it. You would use this regulator to connect the oxygen line to the oxygen bottle.
Unless specifically requested otherwise, the Mountain High oxygen bottles will come with a CGA-540 thread (the USA standard size) in their necks and this is the regulator that will fit onto all of those bottles. This regulator features the SMC quick-lock system for use with the 4mm and 6mm polyurethane tubing.
NB - if you are ordering a German 2L or 3L oxygen bottle you will need the DIN-477 regulator, which is that standard European size.