I made a reproduction of the Commodore 586220 test cartridge. This cartridge was actually used by Commodore themselves to test every unit that came from the assembly line. They called it the “Commodore 64 Final Test Diagnostic Cartridge Rev 586220” and to fully test every single port and function of the C64, a special cable harness is required to terminate all the external ports for a loop test. But this cartridge is still useful without the harness, and it is a great addition to my collection.
The PCB itself is a standard 8K Cartridge that uses a 2764 eprom in address $8000-$9FFF which means there are many games you could burn into the rom, and make a cartridge out of your favourite 8k game to avoid the boring loading process.
You can download the official instruction manual here
This project with PCB and parts can be purchased on ebay: https://www.ebay.com/usr/kirsti_73
or from Sellmyretro.com