not the mystery knob! it was either a monitor or leslie volume control. an input and and an output. as you can see something like this needs 4 jacks. besides if jerry was going to have something like this i am sure it would have just been placed in the rack or better yet in the guitar. also, the mystery knob can be seen by weir when not by jerry or by lesh when not by jerry or weir. it usually follows or is not too far away from the keys/hammond onstage speaker/leslie cabinet.
did i mention this is a great blend pedal!
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.