From Halopedia, the Halo wiki
KEYSTONE was one of several armor projects undertaken by the Office of Naval Intelligence, designed to test the ever-evolving standard for the third generation of powered armor systems. The project was initiated as an evolutionary design study and later combined with elements seen on earlier iterations of the Mark VII project such as the GEN1 specification employed by Naomi-010 and the DECIMATOR for GEN2.
The KEYSTONE helmet was made available for use in Halo 4 as an armor permutation as part of the game's inclusion in Halo: The Master Chief Collection. The helmet is available as part of Season 7: Elite, as an Exchange-exclusive item. The helmet is free to unlock for all players, and costs 0 to purchase. It was never available in Halo 4 for Xbox 360.
List of appearances