By R & S. Bayonet Union Plug, Mark IIIB.  Stores reference 6D/101.  Manufactured in brass and alloy.  Bayonet connector as fitted to the end of the tubes used with the type D oxygen mask.  Also used on the MK.III oxygen tube adaptor hose.  The alloy 'cover' section appears to have been 'gilt' washed and much of this has come off leaving a plain alloy finish. In very good condition with clear King's crown Air Ministry mark.  Stock code O16580.