Here is what Mike says...
'Rivermaster Treble Boost. Vintage Rangemaster Treble Boost style circuit with added tone switch.
At its heart is a vintage Ediswan XB103 Germanium top-hat transistor. The boost control is designed to push your amp into overdrive and enhance the higher frequencies. But, it's not just for pushing the top end.. at the flick of a switch you can add more mids or turn it into a full frequency booster for more low end. The circuit is based on the original Rangemaster and is therefore a positive ground circuit and internal 9v battery powered.
It has also been built on an original style tagboard strip. This pedal tends to work best with amps that are just on the edge of breakup to push them into beautiful overdrive or on a clean amp as a volume boost and tone shaper.'