[PbFut]

Super: your post prompted me to look at my panel closer. They are original. Have the GM part numbers on the back for each color, correct arch to match the door, and also the holes like yours. Looking at the finish closer I now realize that they have been coated with a finish, maybe similar to the material you mentioned. It is evident that whoever did so was very good at masking and/or the coating very thin as you cannot see a line were the insert is. Or the finish is not decaying under the insert. I cannot tell for sure which. Today is a good day to take more time to investigate the panels I have and see if I can salvage. There is no damage at all except this funcky finish. I'll try to document with photos, something I am less than good at, for future reference to others.
I will report back my findings later today.