What is this shim for- 66 Door

I have the windows out of my 65 valiant doors and I have that same piece attached to the vent window where it attaches to the front edge of the door. I took the car apart 3 years ago and I would have attached it there so I would know this is where it belongs. I will find out soon when I reassemble the windows.
JimMac, you are absolutely correct!
The piece in question fits in a pocket in the door where the two vent frame attaching screws, the ones with allen wrench heads, screw into the vent frame.
Thanks to all who contributed to help solve the mystery!