Mask/microphone heater.  Stores reference 6D/909. This was clipped on to the mask-side of the type 26 microphone within a type G mask, and designed to prevent icing over the microphone, reduce risk of freezing of the mask valves, and increase comfort when wearing the mask at very low temperatures.  It is a simple resistance type of heater, rated at 4-5 watts at 24 volts, contains no switch, being turned off by separating the plug and socket. The heater is wired to a 2-pin plug ref. 5c/455.  Extremely rare to find as only in use for a very short period