Drew;
No date given on the material specifications chart. It is on page 204 in my book. Item numbers are given linked to the Feather Model section of the book. The "40 Carbon" is on the part drawing for the barrel lug in this section, page 21. It gives dimensions etc for the lug with a note of the barrel being of "Steel App 40 Carbon" & does carry the '29 date. On the assumption that both of these numbers are correct but reflect different points i time then my thoughts are the 1020 would have been earlier than the "40 Steel". This was most likely still a 1040 steel or similar. Once the steel got up to about a 1025 special process were required for carburizing (Case Hardening) as the core would also harden & then have to be drawn to prevent brittleness which killed the effect of the extra hard case. According to an old Machinery's Handbook tempering of low carbon carburized steel was recommended at 250°F to 325°F.
