That is not a Chevrolet/GM/GMC 1 ton Suburban truck.
There was no such truck model.
It is a standard 1 ton Panel Truck that was converted to have side windows (and rear seats).
That conversion was not done by Chevrolet/GM/GMC factories - it was done by other companies.
Some of those companies might have been listed the the official Chevrolet Truck "Silver Book"
A dealer could arrange for a Silver Book adaptation as part of the original purchase. The standard model was shipped to the "converter's" factory. The conversion was done by that other company; and, the converted truck was shipped back to the dealer, who delivered the converted truck to the buyer.
Members here and at oldGMCtrucks.com have reported that 1-ton size/style conversion on special "crew vans/panels" used by the military, by forestry crews, fire crews, and by crews in oil fields or air ports. Various types of side windows (stationary and sliding) and rear seats or benches have been reported/posted.
If you have seen Panel Truck rear interior side walls and Suburban interior rear interior, you would recognize the Panel Truck interior side walls in photos attached above.
Nonetheless, that is a very nice, very rare, and very useful truck.
What is the code on the Vehicle ID Plate attached to the cab between the driver's side hinges?
And the code on the additional plate, added by the "converter".