This is a 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 data sheet).
The module features voltage controlled resonance.
From Timo Rozendal:
The circuit is the same as the rev 3 of the fonitronik circuit (with the extended cutoff option). Matthias kindly gave me permission to offer this set to you all, he also send me this remark fyi: "just be aware, that the quality of the SSM2044 ICs is not the best. they had quite a bit of variance back then. that's the reason some manufacturers changed to curtis in the 80ies. the issue is that not all SSM2044 will have the same range. "