http://www.lpaero.com/images/cessna1.html
The Original Cessna Design: The center strap provides positive retention of the windshield at the top of the cabin. Positive retention in channel is necessary because this area operates in a negative pressure environment and could "pop out" if not positively retained.
Windshield thickness increases are implemented where the original windshield was 1/8" or 3/16" thick.
So there!