If it's the leading edge(front)of the rear fender on an AD,it's a sheet metal support where the running board bolts through the fender.
AKA rust trap!

It's one of the few areas that was a bit crusty on my rear fenders.
There was some perforation through to the actual outer metal too.
I repaired the outer metal and remade mine and had them spot welded on.


1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.