In the L C Smith Plans & Specifications book, even quite early on, while the bbl steel was not specifically called out it was listed as having approximately 40 points carbon. At that early date it was likely something like a 1040 or perhaps a 1340 (Manganese steel). By the time the Nid came along as Chuck has said it very well may have been made of a higher alloy steel as 4140. In any case 0.40% carbon can lead to welding problems. It is a real shame someone drilled that hole so close to the chamber.
