Good question.
The ATSF book on flatcars and gondolas has several photos of the prototypes. Unfortunately, all the photos are all taken from the side of the car with the sandbags. Part of the mounting that is visible are steel plates that were attached to the wing, and laid flat on the car and were bolted down. I don't have the set, so I don't know if these plates are included. Also can't tell if there is some sort of bracketing on the back side that attaches to the plates, or sits in the stack pockets.
There are a couple of 'bumps' on the top of the wing- that also appear in the photos, and I was speculating those might be the top of supports for the wing, but I don't have the set to take a closer look.
Although one photo shows the cars in a train with locomotive, what I expected to see but don't are cables/guy wires over the top of the load attached to the side of the car. This leads me to the conclusion that either it must have been firmly supported from the back.