What is this shim for- 66 Door

okay- I went to move my door shells for edge painting and while doing the final blow down of dust- this starts all the sudden bouncing around. I’ve seen these before. But not sure what it’s for.
Any ideas?
Thanks- Joe

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The part Mopar Tim and JimMac are referring to is Support 23-64-273 on this page. I have these and it's not the same piece as you have pictured. Note the slots are angled 45*. I'm still trying to figure out where the part you have pictured goes. It was stuck to the inside of the door because I removed all the parts pictured on this page. The guy who beadblasted my doors handed that bracket to me with the doors and had no idea where it was attached. 65 Dart Door parts.jpg