No clue. I've only seen it on one or two other AD trucks online. It does have a center support and it breaks down into three main pieces plus the chromed center edge trim and the brackets

This is when I first got it.
http://farm8.staticflickr.com/7141/6441705645_456298b6de_b.jpg

Here it is installed on the truck.
http://farm8.staticflickr.com/7112/7676714742_90201d1ce9_b.jpg

Here's a giant photo in which you can see the bracket/clamp.
http://farm8.staticflickr.com/7125/7676712288_f646f1900f_o.jpg

1951 Chevy 3100
In the Stovebolt Gallery
www.rustyfiftyone.com