David, you are correct~ the center knobs in each cluster are 4 position rotary switches, one position is wide open, the other three each have a different cap, much like a Vari-tone circuit.
The mini switch between the two volume knobs bypasses the volume knobs, and the one by the 1/4 jack is your mono/stereo.
Inside the cavity it looks like someone penciled in a mark for another potential knob just above the 5-pin jack~ I'm guessing for a LED dimmer.
Mica's file for the bass notes nothing about custom electronics, LEDs or a fretless board. The bridge & tailpiece were most likely swapped at the same time the master volume, CVQ, and rotaries were added.
Daniel, the bass has your typical 5-piece neck. The outer maple laminates match the body really well. Being a '75 model, this bass only has one truss rod.
I know there's at least three previous owners of this bass in the Club. I'd love to hear from anyone else who spent time with #241.