This Aprilaire 4331 Humidifier Water Distribution Tray replacement is used in the following Aprilaire humidifiers: Aprilaire 550, Aprilaire 550A, Aprilaire 550M Humidifier, Aprilaire 600 Humidifier, Aprilaire 600A Humidifier, and the Aprilaire 600M Humidifier. This Aprilaire 4331 Distribution Tray is used with the Aprilaire Water Panel Evaporator and the Aprilaire Scale Control Insert to evenly distribute water within the Aprilaire humidifier.