That is the recirc air damper used in cars with A/C. When you put the controls on MAX that damper closes and the one in the kick panel opens to recirc air inside the car. There is a vacuum line that comes thru a hole in the cowl and connects to the heater control panel to supply vacuum to the damper operator to close it.
It looks like it was cut with tin snips because it was done by GM on their side of the assembly plant after Fischer finished the body.
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