This is a Maintenance kit for Aprilaire Humidifier Models 600, 600A, and 600M. The kits contains the following: 1 #35 pad, 1 #4330 Scale Control Insert, 1 #4331 Yellow Orifice, 1 #4004 Strainer, 1 #4335 Feed Tube Assembly, 1 #4331 Distribution Tray and 1 #4223 Drain Spud.