From Halopedia, the Halo wiki
The armor's helmet was initially created for service in the UNSC Marine Corps, though development delays kept it from consideration by the branch. However, only minor modifications were required to make the HELLJUMPER-class armor's helmet fully compatible with [GEN2] Mjolnir armor.
A small number of HELLJUMPER-class prototypes have been commissioned and entered service into the Spartan branch under a research partnership between the United Nations Space Command and Cascade Stronghold Technologies. Spartans Edward Buck, Kojo Agu, and Michael Crespo used this armor during Operation: COALPEPPER in August 2555. Edward Buck would continue using the armor up to the Battle of Genesis in October 2558.
Halo 5: Guardians
Halo 5: Guardians skins
List of appearances