Features two classic roland resonant lowpass filters, each with a three channel mixer on the input.  Each filter also features a simple hard set high pass filter, 2 CV inputs with attenuators and a 3rd CV input that tracks v/oct for filter tracking effects!  Per Roland:

The 521 Dual VCF (voltage controlled filter) module features two separate low pass filters for modifying the timbre of audio sources. Each filter has its own dedicated controls for frequency cutoff, resonance, and a fixed high pass filter with two switchable cutoff points. Audio and CV input mixers on each channel allow the blending of multiple audio signals and modulation sources.

Note: This is a used item, and while it is guaranteed to be fully functional it is almost sure to also contain "rack rash" on the mounting holes and have some minor scratches on the panel from use. It will include a ribbon cable, m3 screws, and a box, but not the original box.