Design by Sven Petersen.
The Diagnostic Rev. 586220 is a widely accepted test tool for the Commodore C64. It tests the RAM, ROMs and the peripheral chips (VIC, SID, CIA). To test all interfaces, a feedback is required. This is provided by this harness.
SMD version with professionally SMT assembled boards. 3D printed cases. Serial port dongle is also included. I forgot it from the photo.
Cartridge with Diagnostic 586220 software is available here.