Beat frequency oscillators or BFOs first developed from radio telegraphy. The oscillator was used to convert morse code to audible signals.
How does the BFO work?
It has 2 oscillators. One gives a fixed frequency and the second a variable frequency. These two frequencies are fed into a mixer which produces a sum ( we will call this "f1") & difference ( lets call it "f2")
The difference between f1 & f2 always lie in the audio-frequency range.
The demo is in Norwegian (a language I sadly don't understand).
However, I think the video speaks for itself.
Elektronisk urmusikk (8:40 min)
The picture taken below in the BBC shows a device nicknamed the "wobbulator".
The square box with that huge dial.
It's a sine wave oscillator with frequency modulation.... probably a Bruel & Kjaer 1022
The synthesizer hadn't been invented and the BBC along with other studios around the world built their own musical instruments or converted existing electronic test equipment (mainly signal generators and filters) into something that could make sounds.
The central knob sets the frequency of the main oscillator.