So I just got a pair of the Bluford Erie Lackawanna 70-ton hopper cars. They're beautiful! However, looking at proto photos of this class, I note that some have the end-most ribs on each side painted yellow, while others don't. I thought maybe it denoted ore service or something, but that doesn't pan out in the pictures. Were they unit train indications (like the Pennsy yellow ball)? What do they mean, and which cars had them versus which cars didn't? Thanks!!!