L C Smith Plans & Specifications book list their bbl steel as having about 40 points carbon but no alloy number given. Purely a guess but likely either a 1040 or 1340 steel, if I haven't got my numbers confused. 10xx does denote a plain carbon sttel, I seem to recall 11xx is the leaded variety & 13xx is a mangenese steel. This series having a small manganese % was a quite popular bbl steel as I understand. One steel in this category was sold under the trade label "StressProof" & was often used even for high power rifle bbls.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra