Electro-Harmonix (EH) Big Muff Pi Guitar Effect Circuit. The EH Big Muff Pi would probably be better making the change by a modern input-jack power and a DPDT bypass switch.
The Big Muff Pi is a fuzzbox produced in New York City by the Electro-Harmonix company, along with their Russian sister company Sovtek, primarily for use with the electric guitar. It is used by bassists as well due to the Big Muff’s low end frequency response.