This is voltage-controlled low-pass filter with 24dB/octave slope. It is made with the legendary
SSM filter chip SSM2044. This chip was used e.g. in these synthesizers: Korg Polysix, Korg Mono/Poly,
Fairlight II, PPG Wave 2.2 und 2.3, Emu SP-1200, Siel DK600, Siel Opera 6.
The special feature of the 4 pole low pass SSM2044 is the patented so-called "true open loop design
that delivers a characteristic fat sound not available from other devices" (extract from SSM2044
The module features voltage controlled resonance.
The gain control seen on the frontpanel is not considered in the PDF, it's optional - a mod.