Still looking for group/number, but the Data books in 1947, 48, 49, 50, 51, mention the panel with option No.390. It drops off in 52, it lists Deluxe Cab in the index but the page reference only shows corner windows (option No.387). I don't have a '53 book but in '54 and my very rare '54 GMC book (Canada) it does not mention the panel. It looks like it may have been eliminated around '52, a lot of changes with the Korean conflict? It is referred to as the "seat back trim panel." In the Salesman Data Books for those years I mentioned above. At least I can narrow my search now in the parts books.
