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Caltron 6-in-1 is an unlicensed game made for the Nintendo NES. Nintendo never approved the game's release, but the Taiwan based Caltron published the game anyway. Because the game was unlicensed and didn't have wide distribution, there are not many copies in existence.
The cartridge is a compilation of six different games, Cosmos Cop, Magic Carpet 1001, Balloon Monster, Porter, and Bookyman. All six of the games are clones of more popular games, for example Cosmos Cop is very similar to Sega's Space Harrier and Balloon Monster is like Buster Bros.
Although the game is panned by critics for its poor quality, it's rarity make it one of the more expensive NES games. Caltron 6-in-1 sells for about $200.