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!
"The process of selecting the tone on the guitar is an aesthetic process like any other, so you try a lot of different things. Most of the things I’ve tried I’ve tried in live performances" ~ Jerry Garcia