I'll offer that the steel was most likely a plain low carbon alloy, probably similar to SAE 1020 (0.2% carbon). The clue is that the frame is well known to be case hardened. When you hit SAE 1040 (0.4% carbon), you can through harden. The SXS shotgun was pretty well standardized during the time that case hardened low carbon was what was available. It was well known and understood to be sufficiently strong without the case. However, casing added a great deal of wear and corrosion resistance to the surface. Case hardening or use of through hardening alloys added wear resistance to the lever work. Springs, were, of course, high carbon, say like SAE 1095.
