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.
I’ll do my best to keep post going so look out for it. I have a few sets of doors I plan on dissecting to reassemble mine. I also have another formula s 66” I plan on taking apart to see about rear side window assembly.
I may do a post for the help section on all the rollup door and window stuff. I’ll know then.