This is a hand-built device known as a Transient Waveform Modifier. A transient is a voltage waveform that is generated to trigger the envelope of a sound. The envelope shapes the amplification of an oscillator over a short period of time so that it sounds like it has been produced as a musical sound. This is the contour of the sound and is normally thought of as having an attack, decay, sustain and release period (ADSR).
It appears to have been custom built either by or for Tristram Cary and was used in his early studios back as far as, at least, the Fressingfield studio in Suffolk in the UK. It is a black metal panel with sixteen control knobs arranged in four rows of four, with a set of labels between the upper and lower pairs of rows.
Electronically, it consists of four independent channels each of which utilises one row of control knobs. Each row consists of two multi-pole switches and two potentiometers. Each row is numbered, in cream paint, from 1 at the top to 4 at the bottom. In the back of the metal plate are the electronics for the four channels. These consist of the body of the switches and the potentiometers, an EF40 pentode audio amplifier valve, and the necessary passive electronic components to connect everything up and shape the currents so that the device works.
Functionally, this device is a version of the envelope generator used in most analogue audio synthesisers. It has controls:
The left most control is a 6 position switch that sets the degree of differentiation performed on the signal. Differentiation causes the signal to be sharpened up and may be thought of as the "attack" rate (slew rate) of the control voltage envelope. This control sets the rise time or 'attack' of the envelope.
The second control is a three position switch which determines whether the envelope waveform triggered by the transient continues to 'grow' (or rise) or 'die' and drop away (or releases).
The third control is a variable potentiometer that sets the duration of the active period of the envelope, that is, it sets the hold period of the note being shaped by this signal.
The right most control is a potentiometer labelled Low Level Set and is the level of the voltage held during its interim period or the voltage that it remains at between transient events.