[special-K]

I'm not sure what the arched comment refers to. Maybe he meant slanted above the arch. It is slightly slanted in all the same front to back. 'Back then" it was those who cared more than others either about looks or to protect the bed who had tonneau covers. That's what a tonneau cover is and that's the only way they were installed. It was many years later that the modern frame-type tonneaus came to be. GM offered snap on tonneau covers as an accessory. It was not uncommon to see expensive European sports cars, like Porches, to have snap-on tonneau covers installed. It was on open top cars where the concept of tonneau covers began, then carried over to customized pickups later