This is right in the middle of each door glass, across the top edge. It is the hinge where the front third of the top flips over and joins the windshield header. This screw is only present on some cars and not on others. It adjust the top frame so it only opens up 180 degrees at that hinge and don't go any further.
By the way, thanks for the complement on the drawing. I'm a boiler/pressure vessel designer and that's been my "day-job" for 2 decades.