This is a Mountain High CGA-540 (axial-out, XCR, single outlet) regulator used 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.
This regulator comes standard with the Mountain High EDS 1 and 2 person kits and does not need to be ordered as an additional item if you are ordering a full kit.
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.