Have you thought of the possibility that maybe, just maybe, John made up the image? He could have taken images from a couple of ads and then photographed them into one image. Remember the time he photographed a pencil and one of his locos to make a fried think he was actually building in 1/400th scale ? After all John was a photographer, and a good one.
I remember reading (possibly in Linn Wescott's book) that John made his heralds and lettering through photography and tinning down the paper to make it appear as decals. That fact plus his artistic capabilities would make his ability to create the billboard well within his abilities.
Just a thought......
Jim Little, MMR