A diverter damper seal land for installation in a flue gas duct defined by a casing wall, the diverter damper seal land includes: a front seal land and a rear seal land, the front seal land is mountable in the duct by a connection between the base edge and the casing wall and the rear seal land is connected at its base edge to the inner edge of the front seal land and spaced back and radially inwardly from the front seal such that it protrudes radially inwardly behind the front seal land. Another diverter damper seal land assembly includes: a seal land including a front seal land and a rear seal land; a first gusset; and a second gusset spaced from the first gusset, the first and second gussets connected to the casing wall and positioned at the back side of the front seal land to support the seal land in an operative position in the duct.