Nintendo Gameboy Color proto cartridge

A one of a kind item direct from an authorized Nintendo developer (SNK). 
What you see is a Gameboy Color programmable EPROM cartridge which developers use to test their creations. Once they have the programming partially playable, they transfer their programs to the test cartridge, plug it into a Gameboy system (or Wideboy and Nintendo64) and then they can play their work and see how everything is coming along. Items like this are really helpful to programmers and developers. It allows for them to get a feel for the game on the actual hardware and not just on a big computer screen.

In the picture below, you can see the prototype cartridge plugged into a Wideboy CGB with Gameboy attachment unit.
These little open-ended EPROM cartridges cost a cool $90 direct from Nintendo.