Guy, I am intrigued by the wording of the last paragraph of your information (snipped below). I assume from the way you've put it that this was the creation of placarding, and not just the mandating of a convenient method for changeable placards. But is it possible that the innovation was the metal holders themselves?

More generally, what does this innovation say about the way tank cars were used up until the new specification was created? I am guessing it has something to do with a move away from owner-users, to owner-leasers - i.e. that something was changing that resulted in the use of a particular car changing from load to load? (Easier to change a cardboard square than to repaint the car?)

Alternately, or maybe at the same time, could it be that this is when folks realized that greater precision in the warnings was warranted (i.e. an empty car with a residue of product "A" differs in its risk (not always better) from a car containing product "A", and affects how the car is treated in train position, if in a wreck, etc.

Just wondering out loud

Rob Kirkham