I did some for us because I was interested.
I used the Chevy.com tool to "build your own" and got this picture out of it, so yeah, the base "Express Cargo Van" has no windows. (Which kinda makes sense if you are hauling scaffolding, plumbing supplies, etc.)
Vaguely reminds me of:
I've seen them without rear door windows, and I've seen them without rear cargo area windows, but I don't recall seeing one without rear panel windows OR rear door windows.
Especially of the vintage of the hoax bomber's van.
Dunno. Could just be me, but I work around such vehicles all the time, and can't recall seeing one like that before.