I know I had a users manual, for some years ago, I'll look it up and see if I can get a scan of it. But it is not that hard.
The black tiny knob, chooses one or two outputs, so if it's in down position both
piezo and magnetic goes out the same output.
Up position is splitted output.
The metal selector switch chooses piezo or magnetic. This can be easily changed to a three way on-on-on switch so you can run both piezo or magnetic at the same time.
The extra volume knob is piezo volume when split outputs are chosen.