Are those all 6 bolt 15" wheels?
There is a 15" 6 ply (15x5.5 "short spoke steel disc") optional wheel listed for Light Delivery Trucks in the 1939 Chevrolet Salesman Truck Data Book: "low pressure balloons for easy ride and greater capacity"
A 5.50-18 tire option was also available for muddy roads.
The same 15" wheel (artillery style) is shown for the 3/4 ton truck. Maybe someone who has a 30s truck can confirm that the light duty truck (1/2 ton) used 6 or 8 bolt wheels.
