This is a military-style radio headset.
This headset was made using the radio headset from Marcus' mesh, with the earcup duplicated and mirrored.
To take advantage of the bonus effect, both the player and a companion NPC must each be wearing a headset. At that point, the companion gains +5% critical hit chance, to simulate coordinated tactics and communication between the two. It should work with all humanoid companions, even those added by mods, as long as GetPlayerTeammate returns 1. The headset is considered a necklace by the game, so that it can be worn with other headgear.