The subject car appears to be a precursor of the Ulrich car with the cast metal sides. Ulrich was certainly heavily into die casting in the late fifties. I wonder if he was selling non-die cast kits earlier. Like this one.
The "late" stock car (as seen in the sample catalog page) used the same cast side structure as the box car.
Unfortunately, Ulrich used low quality metals, and the models get zinc pest a lot more often than they should.
I sure loved their triple hopper and GS gon, back in the day.