Variants of the American A-13A pressure demand oxygen mask were adapted for use in the RAF with the D-1 demand oxygen regulator.
First ordered by the RAF in 1950 from MSA in the USA.
The number "MS22001" is often printed on the mask rubber facepiece; this was the military specification number issued in the USA for the basic USN / USAF mask before it was Anglicised as the A-13A/1, A-13A/2, and A-13A/4 masks.