Bayonet Union Plug, Mark IIIB. Stores reference 6D/101. By The Automatic Light Controlling Co. Ltd. Brass 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. In excellent condition with clear King's crown Air Ministry mark and 1939 date code. In the original issue box, also in good order. Stock code O20153.