I saw an early 50's Chevrolet flatbed a few years ago that had tandem duals. The first axle was the drive axle and the second axle was a tag. It appeared, from the marks on the frame, that the original axle had been removed and replaced (on a lengthened frame) with the tandem set.
My guess would be the company that installed the fire bed on the truck also reworked the axles to carry the load.
