This is a reproduction Williams Overvoltage board as used on the Stargate power supply, and can be used on the Defender power supply as well.
Williams part number #5773-09664-00.
Details about this board can be found here:
https://archive.org/details/arcademanual_stargate-ds/page/n11
This isn't an exact copy, the pcb routing and size is different as I didn't possess an original board for comparison.
Marc Deslauriers 2019-07-11