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. Cassette adapter has hard gold edge connector. 3D printed cases. Serial port dongle is also included. I forgot it from the photo.