IGS Audio Rubber Bands 500 (RB500) is a passive equaliser, absurdly reduced in size to the 500 series format.
Pultec-style EQ in your 500 series rack.
This tiny module is equipped with transformer balancing on input and output (IGS Custom) as well as a discrete driver circuit.
Of course, the entire filtering section is passive and mirrors analogue equalisers from the 60s. There is a separate frequency selector for low boost/cut sections.
And you get the true bypass switch as a bonus!