The blocking looks correct, Jon, and, as Russ notes, the coils are usually placed over the trucks. A single coil similar to the large one in the last photo in Russ' reply appears to be about 72" in diameter and 48" wide and would weigh around 27 tons. Another reason for limiting them to one at each end of the car is to avoid damage. Most gondolas didn't have cushion underframes in your time period, and slack action can move cargo around if it's not secured. I'd guess that the blocking would be bolted to the car's floor, or, in a steel-floored car, retained by angle irons welded to the floor. As long as the wood is tight against the coil, it's secure. I'd think, though, that multiple coils not separated by solid blocking could move, especially during switching maneuvers. If the edges of a coil are damaged, it's often no longer suitable for the intended use.
Depending on the end use, coils could be covered or not. While I never worked in the strip mill, coils destined for further processing within our plant were stored
directly on the ground, on their sides (as in your cars) outdoors. They might sit for only a few hours, or for weeks, depending on requirements. Further processing usually required "pickling" in an acid bath to remove scale, then, after the processing was completed, the finished coil would be shipped, often in a boxcar. I'd guess that coils travelling in open cars would undergo further processing at their destination, while stuff in covered cars would be, more-or-less, ready to use. Not having worked in that department, though, that's only my semi-educated guess.
Wayne