Is it a filter?
I removed the interior shroud from the original Armstrong AC; just to see what things look like. I can see the squirrel cage blower thru one rectangular shaped hole. There is another rectangular shaped hole toward the condenser end of the unit. When I look up inside, there is some kind of material; can't tell if it is a filter or just sound deadening material. Unfortunately the PO replaced the original exterior shroud with one he fabricated and riveted it down to the roof. I don't really want to remove it if this is not a filter that needs to be maintained. The AC unit works fine.