All The Unico System® supply plenum adapters feature a rectangular to round transition. The UPC-61-X (see table below for part number match-up.) allows air to transition gradually from the blower outlet to the main plenum. The Unico System® Supply Air Plenum Adapter, Round, Series: UPC-61, Supply Air Plenum Adapter Adapter, 7 in Diameter, 8 in Length, 4-3/4 in Width, For Use With: Model M1218 Blower Module, Steel, Hot Dipped Galvanized