This is a Game Boy Advance Wide-Boy designed for use with a Nintendo 64. (Wide-Boys are devices that are plugged into either an NES, SNES or N64 so you can see the Game Boy game on a TV, thus saving developers eyes from having to stare at the little screens all day long). This hardware was manufactured by Intelligent Systems (the company that produced most of Nintendo's official Development Hardware).

Most of the Game Boy Advance hardware is inside the N64 cartridge (there is very little circuitry inside the Game Boy itself, it's basically just a controller). The game can be seen on the GBA LCD screen as well as the TV itself.
Game Boy Advance Wide-Boy in operation.

Inside the N64 Cartridge (where you can see the CPU AGB Arm processor chip):
WIDEBOY AGB(A)

And inside the GBA controller (as best as I can open it):