Woogeroo,

The Chevrolet Truck Specifications Manuals would be a good place to start.

The Old Car Manual Project Specifications Manuals section does not go back real far:

1928 shows no dual rear wheels
(but, larger trucks than Utility might not be listed?)
1937 shows dual rear wheels


This 1936 Truck Specifications clearly states 1/2 ton trucks
This 1930 Truck Specifications shows only single wheel on the 1.5 ton truck

Here are links to other Specifications Manuals - large downloads and a lot of reading/scanning through the pages will be necessary