It's been a long time since I had a stock wing so I have a question. What is this threaded insert used for?

It's the alternative pedestal to go from fastback to notchback and vise versa. The rear stud is in a different position depending on if you have a notchback or a fastback because of the way the under decklid skeleton structure is laid out.

Thank you. I never really paid attention to the notchback wing. That answers my question.