A couple of years ago I drove a Diamond Reo truck just like this one during beet harvest for a friend of mine here in SW Idaho. The beet truck had a large beet box on the back with a hoist so you can dump the beets into a conveyor box at the beet dump. The most weight I ever grossed was 62K pounds, just a little overweight!
It was powered by a Cummins 220 as I recall. Not much power, top speed of 45 on the flat when loaded, almost 55 when empty. It had a 5 speed with a 3-spd brownie. It seemed like no gear was ever quite right, except compound low when trying to get out of the field when fully loaded. You had to drive stakes to judge by to see if you were actually moving, but eventually you could get to the pavement.
I think the farmer still has the truck and it probably worked in the beet harvest last fall. He also has a parts truck that was it's twin. He thought the trucks were originally cemet mixer trucks in the Boise area.
As I recall, it seemed like the cab of the truck was rather small, and seemed to be perhaps a Studebaker truck cab or some such thing.