It was talked about here only with model a's http://www.jalopyjournal.com/forum/showthread.php?t=383063&highlight=war+ration
That addition may not be very "stylish", but has some precedence. Many folks used to add a pickup bed to a coupe in place of the deck lid. And even though apparently home-built, this is a variation of that theme. A pickup bed...with a tonneau cover!
Chevrolet sold factory Coupe-Pickups from 1936 through 1942. The insert above doesn't resemble factory ones I've seen, though, which have the side panel upper section shaped like other pickup beds, angled and rolled with a stake pocket in the end. However my book doesn't show a '39 specifically. The cover for it is homemade for sure. They were just an insert that could be taken out and swapped for a coupe deck lid, too.
It is a thing-a-ma-bob, a thing Bob made to haul more shit than his coupe would haul. Actually not so unusual in rural communities in those days, people made what they had work. Coupe looks pretty spankin clean and that could be taken out and repaired. It actually reminds me of a little girlfriend I had with a little to much junk-in-the-trunk, or a little more cushion for the pushin!! ~Sololobo~
I looked up the CL ad, looks like a really nice car for $3,500. Wouldn't be a big deal to remove the box and replace it with the deck lid...as I recall the opening in the body remained the same.
Found the ad here. Unless the deck lid went all the way to the belt line though, it may not be so easy to just put a new deck lid on it. Seems like a nice old car for $3500 though.