The Fort Plum plaque is a plaque contained in the Rikti War Zone co-op zone. It is not currently associated with any known history badge.
Fort Plum: Forewarned
This plaque is in one of the Vanguard forts. Other forts in the zone have similar plaques.
This one can be found in the fort of Point Du Hoc neighborhood, at the fort where the players will be teleported at the end of a Rikti Ship Raid.
Its coordinates are (4100, 6.2, -3843.5).