Chevrolet introduced step-van chassis in 1948. Several different manufacturers made bodies for them so you might find ten or twelve vans all built the same year but might all look a little different from the rest. Try to find a manufacturer's ID plate somewhere on the van. Maybe on a door edge or a door jamb. If you can find the VIN number it will be much easier to determine when the van was built. Maybe someone else can tell you specifically where to find the ID plate.
