Bro. Nudge: A sample of 5 certainly does not establish statistical significance, but AFAIK this is the only analysis of U.S. maker’s frames anyone has done

c. 1890 Hunter Arms Fulton “Transition” L.C. Smith Hammerless and 1909 L.C. Smith No. 00 were both non-standard but similar to Bessemer resulphurized (very high sulphur) AISI 1109 low (.09% & .12%) carbon steel.
Plans & Specifications of the L.C. Smith Shotgun by William S. Brophy contains an undated but likely post-1913 Materials Specification chart indicating “AISI 1020 Carbon Steel” for the frame.

c. 1900 Remington Hammerless Model of 1900 was Bessemer rephosphorized resulphurized AISI 1211 low alloy low (.08%) carbon Steel

c. 1892 New Ithaca (Hammer) Gun was AISI 1015 carbon steel

c. 1927 Crescent Empire No. 60 was non-standard AISI 1015 carbon steel with low concentrations of nickel (.05%) and chromium (.08%)

c. 1929 Fox Sterlingworth was non-standard AISI 1020 with low concentrations of nickel (.07%) and chromium (.08%)

Possibly someone could check Walt's book and see if the Flues era frame composition is specified.

I don't know if frames were heat treated prior to the color case hardening process, or if that was considered as heat treatment. If improperly done, heat treatment can decrease ductility and increase brittleness of low alloy steel.