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 more elements you have stable, the more you can concentrate on your playing and not be continually adapting your technique to your equipment." ~ Jerry Garcia