Portable Army Radio Tested (Nov, 1937)
It looks like you should be able to wind up that key in his back and make him march.
Portable Army Radio Tested
A PORTABLE field radio transmitting and receiving set that operates while strapped to a soldier’s back was satisfactorily tested by the Royal Corps of Signals at Alder-shot, England. The device features a special loop-type antenna, standard earphones and a hand microphone. The power supply unit is self-contained.