Note the size compared to the nickle.
"ARP 4075 Resonant Analog Filter PCB assembly
Original ARP circuit board assembly pulled from a working OMNI many years ago. This is the same board which was featured in many ARP classic synthesizer designs, including the 2600, Odyssey, AXXE and others. With just a few additional components this could PCB could make an great addition to a DIY modular synthesizer, replacement for a bunk potted 2600 filter module, or even a standalone effect unit. The board requires the standard (bipolar) +/-15 Volt power supply and a few resistors, pots, and trimmers to be used as a standalone device. The board features several long pins which allow it to mate with another board (on which the pots, etc may be mounted), making it easy to work with."