
The Sundt Engineering Company of Chicago was advertising this kind of device in the June 1936 issue of Short Wave Craft Magazine. Only two dollars (but that was big money in 1936). From Bob's article: "When the transmitter is modulated with a single audio tone, the waveform of the modulated carrier will be seen. By varying the motor speed (horizontal scan rate) the pattern can be synchronized or made to stand still. Percentage modulation is readily estimated by simple inspection of the display."
So here we have the perfect minimalist 'scope to go with a minimalist AM transmitter!