I have recently come across a 1960's DeArmond Pedal (Volume?) Model 602 S1. It only has one input on the left side for a 1/4"plug. Other similar types I've seen on Reverb and Ebay all have an instrument input and an amp input on the opposite side and none have the S1 designation. I'm wondering if anyone out there can shed some light on this pedal. Specifically, what was/is it used for and how can it be used with only the one input? Thank you in advance. C.