The ArmetX - Jolly Jack Dark was originally a reward version of the Overmatch ArmetX that features the same stats.
This helmet could previously be won through the "RARE HELMET LOTTERY BOX", which was available post-Parity Patch, untill it was removed in the holiday update on the 18th of december, 2015.
This helmet was handed out as a 1 day version to all players who logged in after the Onslaught patch.
In-game Description Edit
Standard helmet. A Jolly Roger adorns this version of the Overmatch ArmetX