Nintendo Wideboy 64 GB
Developers tool to play and debug GameBoy cartridges through a Nintendo 64 system

This is the very rare developers cartridge of the original Wide-Boy 64 GB made by the only authorized developmental tool creators for all Nintendo items, Intelligent Systems. Only a small hand-full of these original Wide-Boy 64 GB units ever got distributed because almost immediately following this model, the newer Wide-Boy 64 CGB units (which also worked with color gameboy cartridges) were made and sold to developers.
This cartridge would plug directly into a Nintendo64 game system to allow for Gameboy cartridges to be displayed through a full monitor and which could only be controlled by using the Nintendo64 controllers. This would allow for more people, developers, producers, and decision makers to see and experience the  game play and action in a larger presentation. These units would also allow for individual sprite manipulation as well as better screen shots to be taken. These are also used for game shows in which the companies would want their games to be displayed on larger monitors so more people could see the game in action. 
These units could only be bought directly from Nintendo at a cost of $1400.00 each, if you were a Nintendo authorized developer or magazine publisher.