That DF was a trademarked load restraint system, and the NYC box is likewise marked "SL". That was another of the same sort of system by a different manufacturer, standing for "Secure Load".

I'm betting the various components weren't interchangeable, so not only were the cars labelled so the correct anchors and bulkheads were used in each, but never hurts to have a little advertisement in a contemporary font out there for all to see.