This is a Game Boy Cheat Device, similar to a Game Shark or a Game Genie. It was made for the Taiwan game market.
 
The unit is technically new, unsold stock, but due to age it's not 100% mint or anything. There's a cardboard insert inside that has some foxing on it, sort of like how old books might get those brownish / yellow spots, but it's not moldy and doesn't have a bad smell or anything.
 
Everything is tested and works fine