It is a 1950 GMC half ton. (Vintage Truck August 2007, page 49) Maybe the word 'valance' was wrong now that you mention it. I know there is a valance on the front. What I am referring to is the vertical sheet metal below the tailgate which might be part of the rear bed sill. I have a hole in the center of mine. The photo in VT shows a rod running between the two holes. Now whether it is original or just something the owner added, I can't say.
