This is a BRAND NEW OEM 2 Pack of Aprilaire Humidifier Water Panel Pads.  This is not a knock off like most online sites sale.  The Aprilaire Stock no. is 45 and the water panel measures roughly 13-1/2" x 10" x 2"
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This Aprilaire Water Panel works in the following  Models:

Aprilaire Humidifier Models 400 400A 400M