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

View attachment 1715205028

View attachment 1715205029
Please let me know when you find out for sure where that piece goes. Putting my doors back together and I have one of those that came off during bead blasting.