Hand-made in New Zealand by Paul Crowther, the Prunes and Custard is a small pedal with a lot of capabilities.

There is a preamp stage, the gain of which is adjusted with the Drive control. This stage followed by a special waveshaping distortion circuit. There is some treble roll-off between the preamp and the special distortion circuit. The frequency at which this treble roll-off happens can be changed with the Guitar/Bass switch.

The special distortion stage produces strong harmonic overtones, mostly odd harmonic. Higher harmonics are progressively introduced with increased Drive settings and signal level, so a decaying bass guitar note can sound like a mean swept filter. Playing double notes, like 4ths or 5ths can produce some strong intermodulation effects, rather like a ring modulator, but more musical, albeit more distorted! 

The Mix control is a balance between the preamp stage output, (CCW) and the special distortion stage output (CW).