The brass button is a type of trinket awarded from Imperial outposts to some experienced pirates1, randomly chosen from those aboard the ship, when they defeat the soldiers in the swordfight. It was first seen in release 2010-03-09. It is one of the more common types of Imperial outpost trinkets. As with all Imperial outposts trinkets, the brass button is a two-color trinket, available only in the colors of the Imperial outpost palette.
1 On doubloon oceans, the player must be holding at least a Pirate badge. On subscription oceans, the player must be subscribed.