This i found as a scan from some ye oldee electronics magazine. I have no idea what it does, but text from the article made me draw it :)
Excerpt from the article:
Fuzz, echo, reverb, big bass, they're all out! The new guitar sound is funky, and you'll get that with the wide range funk box. Just crank potentiometer R7 (range) and you'll get an extra twang from way down low to way up high.
Adjustment is easy, simply vary potentiometer R4 (trimmer) until you hear a whistle (oscillation); then back off R4 until the oscillation just ceases.