As I removed the vent outlet in the BOTH of my doors I found this:
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In the picture you can see the vent outlet (#100761) from behind. In my hand I am holding a little flexible rubber mat and on the desk is a hard plastic piece that is identical to the door that is already installed in the vent (opens/blocks the air flow). Both of these pieces were just laying there in the duct and I assume they are meant to help sealing the vent door. I also assume they should be sandwiched like this in the vent: door#1/rubber piece/door#2. Am I correct? If so, what is the best way to reattach the loose door and rubber piece onto the door in the vent? Glue?