There was an extra pair of wires attached to a circuit board behind the knobs which then lead to a nine volt battery. Obviously someone had added a pre-amp. This pre-amp has a normal volume knob and a second knob which consists of a pair of concentric pots. The inner pot has a bent shaft and has no knob and I'm not even sure what it does.

The circuit board has the words "WY Harmonics Booster" embossed on it. I don't know anything about this particular mod. I'm hoping someone out there might be able shed some more light on this for me.