From Halopedia, the Halo wiki
The MJOLNIR Powered Assault Armor/B variant refers to two distinct sub-models of MJOLNIR Powered Assault Armor, created for the Mark IV and Mark V generations. The latter iteration of the [B] variant served as the initial production model of the Mark V MJOLNIR Powered Assault Armor.
While its physical architecture is derived from the Mark IV(b), the Mark V[B] was the first mainline model of the MJOLNIR Powered Assault Armor to incorporate energy shielding technology. The Mark V[B] is largely similar to the final version of the Mark V. The only major difference between the two is the [B] variant's lack of an accompanying upgrade to the wearer's neural lace enabling the wearer to interface with a smart AI. This did not come to fruition until John-117 and Cortana successfully tested the final Mark V on August 29, 2552.
Along with the Security and Rogue variants, the Mark V[B] was designed and manufactured by private organizations hired by the UNSC rather than developed by the Materials Group due to numerous problems with funding.[verification needed] The development of Mark V[B] was conducted in secret, with its prototypes tested by Spartan-III personnel. The physical architecture of two later MJOLNIR variants, Mjolnir Mark VII prototypes and DECIMATOR-class Mjolnir, closely resembled the design of Mark V[B].
The Mark IV(b)'s first known use was by Frederic-104 during an operation against Insurrectionists sometime before the Battle of Circinius IV on April 26, 2526. Fred continued to wear the armor during this battle when Blue Team was sent to evacuate the survivors from the Corbulo Academy of Military Science. The Mark V[B] entered service with the Spartan-IIs and select Spartan-IIIs on November 24, 2551. The Mark V[B] helmet's only adopter was SPECWAR Group Three. Mark V[B] was the standard Mjolnir variant issued to Spartans assigned to SPECWAR Group Three, including NOBLE Team. While most members of NOBLE Team modified their Mark V[B] suits to varying extents, Thom-A293 wore a full, unaltered set of Mark V[B] armor. Thom's replacement, SPARTAN-B312, also wore a largely unaltered set of Mark V[B] armor. A replica of B312's helmet was later hosted for display in the Outpost Discovery program, following the war's end.
Because the Mark V[B] helmet was not used outside SPECWAR Group Three, the standard helmet worn with the Mark V[B] was instead based on the architecture of the original Mark IV model. This would later be used as the basis of the final iteration of the Mark V helmet.
The Mark V[B] is the default suit of armor in Halo: Reach. The second generation of the Mark V is seen in Halo: Combat Evolved. This helmet is the starting helmet for all players, and is purchased and equipped by default. The helmet is the only component of the set that can be specifically selected in the armory. In Halo: The Master Chief Collection, the helmet variants are attainable in the Season 1: Noble progression system, with an additional robot arm variant of the chest introduced in Season 5: Anvil.
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