I've wondered about that too. I've got three Grade 4E NIDs. The 20-gauge was shipped in 1933 and on the bottoms of the tubes says Best Fluid Steel and also had the big D with a three-lobed crown over it that was the mark of Didiere barrel works of St Etienne, France. My 12-gauge was shipped in 1936, and just has Best Fluid Steel on the bottoms of the tubes. My 16-gauge was shipped in 1941 and the bottoms of the tubes are unmarked.