via this auction
Appears to be SN 1506 ?

Modifications:
Fit prestopatch socket
Stabilisation for Osc 3
Dual transistor upgrade for Osc 3
Oscillator Sync on Osc 2
Hi/Lo frequency range Osc 3
Voltage controlled shape Osc 2 & Osc 3
Portamento / Glide
Attack time extension
x10 Envelope attack / decay time multiplier
VC Filter response
10-Turn pot input Ch1
Patchable voltage inverter
Filter mode switchable 18/24 db/oct
Filter "unslew" switchable on filter's CV
Main signal output mutes switches
External gate input - buffered
Extra matrix pins
The VCS3 has three oscillators (the first two oscillators are normal oscillators and the third an LFO or Low Frequency Oscillator), a noise generator, two input amplifiers, a ring modulator, 24 dB/octave voltage controlled low pass filter (VCF), a trapezoid envelope generator, joy-stick controller, voltage controlled spring reverb unit and 2 stereo output amplifiers. Unlike most modular synthesiser systems which use cables to link components together, the VCS 3 uses a distinctive patch board matrix into which pins are inserted in order to connect its components."