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, 8 in Diameter, 8.68 in Length, 8 in Width, For Use With: Model MB3642x Blower Module, Steel, Hot Dipped Galvanized