From Halopedia, the Halo wiki
The COS-class bears a notable resemblance to the GEN2 models of the GUNGNIR-class Mjolnir, sharing near-identical torso chassis', shoulder pauldrons and forearm gauntlets.
COS-class was built in the skunkworks departments of ANVIL Station. The suit was also tested on ANVIL Station, where the station's cyberintrusion specialists used the armour to test experimental counter-intrusion modules.
The COS armor set was originally created as a useable armor permutation for Halo Online, a cancelled Halo 3-derived multiplayer game developed by Saber Interactive and Innova Systems which was intended to be released exclusively in Russia for the PC. After the game's cancellation in 2016, the armor was one of six sets added to Halo 3 in The Master Chief Collection as part of the game's sixth season in 2021.
The suit set bears a notable heavy resemblance to the GUNGNIR-class Mjolnir seen in Halo 4 and Halo 5: Guardians, with the suit's internal game filenames even referring to it as such. In the original Halo Online game, the suit was known as GUNGNIR in the Alpha builds, though was later changed to the name of Kaladbolg.
In the real-world, COS is a military acronym, standing for Cyber Operations Specialist - the role employed by the COS armor.
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