Product review (5 stars): Exactly what I was looking for
I bought this part so I could have my Whirlpool microwave vent to the exterior of my house instead of re-circulating the air. The microwave was already in the house when I moved in and didn't come with the spare parts.

This part was $60+ at local spare parts stores. The item number on the product name is correct and this is exactly what I was looking for. I was a little apprehensive to buy it since the description only says it works with Maytag microwaves but is labeled Whirlpool. Rest assured, this is the part if you found it via the part number 8206442. The dimensions are roughly 3-1/4" x 10".

Attached it to the back of the microwave and turned on the vent fan. The flapper lifts up and comes down to cover the microwave vent holes once the vent fan is turned off. Did this a couple of time to try to get it to stall in the up position, but it didn't.

CAVEAT
This part does not come with the two machine screws you will need to attach it to the microwave.