Bongo bass piezo control – but not stacked knob?

Hi all! I recently got a ’13 5 string fretless Bongo (lefty), and I opened up the cavity and lo and behold, there’s a piezo circuit in there! However, it has the standard knob configuration (volume, pup pan, stacked mids, and stacked treble/bass). I haven’t a clue how to activate the piezo. Does anyone have thoughts on this?

Thanks!