I bought a PCB of the Weird Sound Generator from the excellent Music From Outer Space recently, and it finally arrived!
The unit is made up of two square wave oscillators (made using Schmitt triggers). These are modulated by an LFO, which is basically another square wave oscillator built using a Schmitt trigger configuration, tuned to operate at a much lower frequency than the other two. This then all runs through a low-pass filter with a variable cut-off frequency.
I decided it would be more fun not to label anything and just have some craic with it. You can get the PCB for pretty cheap if you go here