This is a basic example of a 2600 diagnostic cart. Standard verbiage on label, etc. of “2.6D Diagnostic Cartridge, Stand-Alone, For use with NTSC games.” This is a nice pic, as you can see the board and chip. The scotch tape and yellowing label tells me this one got a lot of use! Has “2.6 DOM” labeling on the eprom chip. There are lots of these out there and I would not say it’s super rare, but it *is* super meta.
“The 2600 Diagnostic Test Cartridge (version 2.6 DTC) contains a variety of tests to assist the service technician in identifying the source of problems within the VCS switchboard and motherboard hardware.
The tests available in the cartridge are:
Color Bar Test
Gray Bar Test
Diagnostic Matrix Test
Audio Tones Test
Paddle Control Lines Test” (source: archive.org)