[Arran]

Arran, this type of German radio lamps has another slang name - "helmets". From the extraordinary similarity with the German helmets of the Second World War. And you would probably be very surprised if you saw the structure of the armature of these radio lamps - it is not placed vertically, but horizontally. That is, the filament, cathode, grid and anode are in a horizontal position, not in a vertical position! As it was in radio lamps at the beginning of the 20s

Peter;
I could see one adopting a slang term like that for those tubes, we typically call those German helmets "Sergeant Schultz" helmets, or "Coal Scuttle" helmets. They are a pretty decent design for protecting your head, though very heavy compared to a British or Canadian "Tommy Hat" style helmet. They were popular in use as motorcycle helmets at one time, though now they make lighter, and safer replicas out of fiberglass, which have padding in them.
I forgot about the internal structure of those tubes, I think that I read about that elsewhere. It was rather retrograde, the only tube that I have personally own with that sort of arrangement was a Myers tube, which was a triode intended for D.C operation, that produced in Canada in the early to mid 1920s. I wonder if the stahl tubes have more problems with shorts between the elements, anode to grid, heater to cathode, etc. ? The horizontal structure would explain the pin arrangement, the heater, and cathode connections would come out of one end, the grid and anode connections out of the other. The footprint of the socket seems wasteful, one would think that they would make use of the larger diameter to use with multi section tubes, by adding extra pins, but apparently they did not. I think that the Soviet electronics industry was more sensible in adopting octal based tubes, and later the all glass 7 and 9 pin ones ( i think they had lock in tubes as well did they not?) 
Regards
Arran