Another Photo ID Challenge! Name the brand of support truck associated with the 1939 Boyle Racing Team

From Paul Lashbrook:
I believe the attached photograph is from 1939, when Boyle Racing Headquarters owned the winning Maserati race car driven by Wilbur Shaw who was the second person to win the Indianapolis 500 three times, and the first to win twice in a row. His car number was 2. There are two cars numbered 2, so apparently one was a backup car. Car number 4 was probably for a team mate. The 2 car on the ground is sleeker. None of this will help identify the truck.