The Scale Control Insert 4330 is the u-shaped component that encases the Water Panel Evaporator. Used with water panel number 35 and the water distribution tray number 4331. Referred as part number 4839 maintenance kit. 

.
The Aprilaire 4331 Water Distribution Tray distributes water evenly to all portions of the water panel. It attaches to the u-shaped scale control insert, part number 4330 for the model 600, 600A, and 600M, Used with water panel number 35 for the 600 series.

Features:  

Dimensions:  11.25 x 15.75  inches  (W x H)

 For Aprilaire Models 600, 600A and 600M.

Also see the part number 4792 maintenance kit for the 500 series, the u-shaped scale control insert part # 4640 for the model 500, 500A, and 500M. Used with water panel number 10 for the 500 series