V320 is the van version of the 320, however all doco / pics I've seen show them as a van with windows.
earlydatsun.com is a good web site for older Datsuns, it doesn't show a windowless version and seems to say they all had a back seat - http://www.earlydatsun.com/datsun32063.html
Maybe it was "converted" for use as a mail van rather than sold that way by Datsun (same way F100's, etc, were converted for use as ambulances) ? Not sure if its just me, but the side panels look a little amateurish at the front where they meet the door jam ? Possibly rear seat was completely removed and area where it would normally fold down was converted to your "secret compartment" ? I'd have a really good look in these areas to see if it looks like a "factory" finish or something done afterwards.
But yeah, pretty rare beast. Value ? Rarity doesn't always translate to high value - 320's aren't the most popular thing around, if I had to guess, maybe a grand or thereabouts ? Maybe more since its a runner ?