The filename is usually something like Halo_2600_Prototype.rom .
The evidence? A single, blurry photo posted to Usenet in 1999. The post read: "Found this at a garage sale. Looks like Halo for the 2600. Anyone have a Supercharger to test this?" halo atari 2600 rom
No original Bungie prototype exists. The processing power simply wasn't there. The "garage sale" find was almost certainly a photoshopped image of Robot Tank or Battlezone . The filename is usually something like Halo_2600_Prototype