Rock Island Aluminum Box Car

I found an old Athearn metal kit for Rock Island 40' single door aluminum boxcar #20060. This was apparently a one-of-a-kind experiment in 1945 (the built date on the car side) to increase capacity (the car sides are 8" taller) and reduce weight over typical PS-1 cars. The kit was assembled well, and only needs the roofwalk end braces reattached and a new brakewheel. However, the underbody is not finished or painted. As I restore the car, I wonder whether this car ran on a typical steel frame, or whether it perhaps had aluminum frame parts? Was the underfame also aluminum color? I notice the bottom edge of the sides with the tabs over the ends of the frame ribs was painted black by Athearn, suggesting the underbody perhaps should be black. Does anyone know of photos or reference articles on this car? GQ