Immediately post-WWI “Fluid Steel” barrels appear to be primarily AISI 1030 or 1040 Carbon Steel, possibly rephosphorized, with alloy steels appearing on higher grade guns; Krupp and Winchester Nickel Steel, Poldi Antikorro, Böhler Antinit.
I may have missed it, but have you tested any of the above? I was interested in Poldi Antikorro and recently read of the following values, which may be old hat:
"Poldi-Anti-Corro" patented in 1910:
....contained 0.145% Carbon, 0.45% Silicon, 0.50% Manganese and 9.78% Nickel - cited under -C. C. Leidedera-1915?