This is a Mountain High DIN-477 (axial-out, XCR, single outlet) regulator used to connect the oxygen line to the oxygen bottle.
This regulator features the SMC quick-lock system for use with the 4mm and 6mm polyurethane tubing.
DIN-477 regulators are for use with the German 2L and 3L bottles which have the European DIN-477 size thread. Some Mountain High bottles can be ordered with a DIN-477 thread, but unless specifically requested they will come with a CGA-540 thread.
This regulator will come with the Mountain High EDS 1 and 2 person kits if you are ordering a German bottle.