The North American Industry Standard for oxygen fittings are called ‘CGA-540’ (Compressed Gas Assn. fitting No.540). Aircraft with built-in oxygen systems do not usually use the heavy fittings, instead special lighter weight fill fittings have been developed. We have transfiller fittings for easy refilling your built in system from standard CGA-540 oxygen systems. The adapters shown are for the Aircraft fillers. The (N) is for ‘Nipple’ and the (S) is or ‘Socket’ on the CGA-540 end. An industrial cylinder has a socket. Micro particle filters are used and an O-ring is on the 540 nipple for an airtight fit by hand.