Scott 9/16-18 style Bulkhead fill Stations.
There are two types of aircraft oxygen filler fittings found in most GA aircraft. The earliest and most common versions are known as the MS22066 & MS22035 (Shown left). They are a 3/8-24 threaded fitting with a small valve-core or poppet valve for a check-valve that opens during transfilling. The other is a larger 9/16-18 thread commonly known as the SCOTT (26875), Airline or AS1219 fitting