Info from the manufacturer about the original:
The "Mod" Sound For Any Guitar Amplifier. Just plug this new unit between your guitar and your amplifier and you've got that "fuzz" sound. Built into a footswitch housing, this new "Fuzz" Booster is an all-transistor circuit powered by its own internal battery. Push the switch to add the "fuzz", push it again to play it straight. Two controls permit you to add the degree of "fuzz" you wish and to alter the character of the tone as you like it. The circuit uses a 1.5V AA battery (not included) and since so little current is required, the battery lasts a long time ... the circuit is turned on and off automatically as the guitar cord is connected or disconnected. The die-cast metal housing will withstand years of use and has a hinged base plate for battery replacement. Build it in one evening and increase the versatility of your music system.
Modded version with volume trimmer (top) and supply trimmer (bottom) to fine tune the 1.5V supply.