The A1 in your body number means it was sold as a chassis-cowl unit without a full cab. Most of the REA trucks from all manufacturers came that way -- the body was built by a separate company.
Your truck could have originally come with either 17" or 20" wheels. Tubeless tires/wheels were not available until the 1950s. It's possible that in the past 60 years, someone has replaced some of the "widow-maker" split rims with the more modern tubeless wheels. Many of the bolt patterns are the same. There would be no problems as long as all the wheels on a given axle were swapped at the same time and differences in hub-centered-vs-stud-centered were accommodated.
Posted on Feb 5, 2012, 8:19 AM from IP address 18.104.22.168