This is a custom overdrive built for master horsetradesman Erick Coleman.  Its a standard diode-clipper type of overdrive; the same mechanism used in pretty much EVERY overdrive ever made from Klon to Tubescreamer to Guv’nor to Rat.  This particular one is based on the most simple of those clipper circuits, the old Electra Distortion.  It has a single transistor providing the gain, and then switchable diode pairs which clip the signal for the overdrive effect.  Each switch controls the switching diode in one direction, and switches between a silicon diode, no diode, or a germanium diode.  This allows Erick to to jump around between a lot of clipping, a little clipping, no clipping, and any asymmetrical variety in between that suits his ears.  Pretty dang cool.  The controls here are gain and output volume