Pickup bed ID

[wrndln]

There is a pickup bed listed on eBay currently. I downloaded one of the pictures from the listing. I have never seen a bed quite like this one. I looks like a regular model A pickup bed, but the lower rails have outward bulges under where the fenders are located. I have two beds and both have totally straight lower rails. I can't remember ever seeing a bed with these bulges in the lower rails. Can anyone explain when the bed in the picture was used? The link to the listing is shown below.
Rusty

http://www.ebay.com/itm/171925220834...%3AMEBIDX%3AIT