I have a '67 K/10 Sub with lift hatch and tailgate. I have not memorized my SPID, but I think it was the other way around. If you wanted Barn Doors, you checked that option. I can't remember the official GM term for Barn Doors, either.
Panel Trucks did normally have standard Barn Doors. I don't think I've ever seen a Panel Truck with a tailgate.
Ambulances had Barn Doors.
A shot of my SPID.
No RPO for either tailgate or barn doors. One would be Standard, the other would be an RPO. Since it was a pretty basic model ,I would guess barn doors would be the option on a Suburban. Or course I have been wrong numerous times..
Getting back to the OP's subject of cruising with a Suburban: That's what I bought my Sub for -- working long distance locations. I had a '68 Shortbed Stepside, with a big crossbed toolbox on the back. Just not enough room in the bed for all my tools and gear, and then there was the security issue. Anyone with a Buck knife could rip up my tarp and get at my stuff. With no back seats, there was a lot of space in the back. Above the gear on the floor, I hooked a hammock -- strung diagonally from Passenger side B pillar to an unused barndoor bolt hole on the Driver's side. Sleeping in it was comfortable in the summer. In fact, as a test. I rode in back in the hammock while a friend drove around some residential hilly backstreets. When the truck turned left, my body swung right and vice-versa. Probably suicidal at freeway speeds -- and surely illegal -- but it was groovy. With a BBC 454 it was a thirsty beast, hence the nickname ''White Fang.'' But it would go anywhere.