I've never seen that assassin chest piece before. But there haven't been any new armors for the class added to the PvP listing since the last event. Some of the possibilities are A) A new standard set of armor has been added to the game recently, but not the PvP armor list, B) One of the previous armor sets was replaced with a new design, or C) This is a set of PvE armor.
Just throwin' it out there. Peace and chicken grease.