[Lynn Eades]

If you look at the box that supports the passenger seat, there is a tag riveted to it with the vehicle info on it. An "S" Bedford is only rear wheel drive. An "R" Bedford is 4wd. The models for the "R" series were: RLB, RLC, RLD, RLF, RLHC, RLHZ, and RLW.
As will your specimen, the Front wheels are bigger in Dia. than the rear duals. This is as per the N.Z. Army wreckers, and so a different rear axle with a suitable ratio to match with the front diff. I would guess that this would make the truck in question an RLHC or an RLHZ. That's the best I can do.
As I recall you can see the plate when you open the door.
The "S" models sit much lower and without looking, the headlights might be situated at the top of the headlight mounting plates? I haven't checked.
Grant, I very much doubt the cab width has changed. The rust marks on the roof, I cant explain, but the rest of the cab is std.