That is the MERV 11 replacement clear out for Aprilaire Air Cleaners. The clear out is the guts of the air cleaner and must be modified every year. Use most effective genuine Aprilaire SelfSeal #210 to make sure the most productive performance of your air cleaner.
Fits models 2200, 2120, 1210, 2210, 3210 and 4200
Requires annual replacement