This cool little gadget is called the Atari Cat Box. It's a testing system that Atari sold to arcade vendors back in the 80's to test their arcade games. If you have an Atari arcade game from the early 80's, you've probably noticed an extra edge-connector on the motherboard- This is one of the items made to use that connector. It can basically communicate with the system board and report back where problems are occuring. (It's not just that simple really, you have to connect the wire probes to various points on the board and read the little display for information, etc, etc... But it does make it easier than just trying to use a logic prode or scope.)

Here's the outside of the little case it's contained in:

And a closeup of the 'controls':

There's also another adapter called the Z80 Interface. This is used specifically for games that have a
Z-80 microprocessor in them. This interface would actually be plugged into the board in place of the Z80 chip, and then plugged into the Cat Box.