This electrical plug and armored cable came from the Paul Strähle collection.  Strähle was a pilot in the German Air Service during the First World War and operated several aviation-related businesses after the war.  Electricity, often generated by a small propeller-driven generator, was used in early aircraft for heating flight suits, for instance.  The drawing below is from a British Ministry of Munitions Technical Dept. Report, I.C. 626, on a captured Hannoveraner Biplane dated July, 1918, and shows the wiring for this type of electric service.  The photo of an aircrew member shows him in a heated flightsuit which he would plug-in to a plug just like this one.  The next photo shows aircraft radio equipment with a similar plug device (page 275, Funkentelegraphie fur Flugzeuge).