I see in the GM assembly manual these things called "fillers". Over at the Goodguys I was able to see down into this area on a car and it looked like there were some dense foam pads at this spot. Is that all these "Filler" amount to, sort of a pad where the tack ("trim") strip screws to the body?
Next, it shows this measurement for the rear bow. It's extremely clear, 17 and 5/8 inches from the inside of the rolled edge on the rear bow to the center of the next bow. This is perfectly clear, make a stick that goes here to hold it and done deal, put the rear curtain in. That's how I see it at least.
But then they give this very elaborate directions for making a stick that would go there and it's adjustable! Why the heck would it need to be adjustable if this measurement is so defined and clear?