This is one of the more interesting Atari collectibles. Coca-Cola commissioned a game from Atari to give to their Atlanta employees. In this case, Atari redesigned Space Invaders so that you shoot the letters "P E P S I" instead of space creatures. There were 125 copies of this game made. There is no real box for this one, just a flimsy Styrofoam shell. So it isn’t really a prototype, but it wasn’t a commercially available game either. And no, Coca-Cola does not have any copies left.
