On the angled lip above the windshield there are a string of pre-drilled holes that I mounted a rear-view mirror from my '28 Model "A" open phaeton. It has just the right amount of angle and length from the bracket to fit the same eye vision for seeing straight out the back window. I will try to post a picture.
Perhaps a previous owner drilled the holes but I have mounted my mirror and air-conditioner from the header brace. It's a mirror used on the "open" Model "A"s and I can see just fine outside the rear window. The black mounting bracket comes curved to fit the windshield frame but I straightened it out in my vice.