The EQ is made up of two transistor gyrators. These are the building blocks of most graphic EQ circuits.
There's a good bit of info available about them, and Jack Orman even has a nice calculator tool to work out the resistor/cap values needed for a particular center frequency / Q / gain.
Guitar Pedals: Gyrator Filter Calculator
Calculate freuqency cutoff for guitar effects pedals with vacuum tubes and solid state electronics. FREE calculator.www.muzique.com
This suggests that the Sherwood's treble-cut-only EQ is different from the Westwood.Bass: Selects amount of low frequencies around 80Hz. Clockwise to boost, counterclockwise to cut.Treble: Selects amount of high frequencies around 2kHz. Clockwise to boost, counterclockwise to cut.