Drew do you have any definitive reference on exactly when the AISI nomenclature actually began?
Best I can find is a vague 'early 1940's'.
Hatcher in 'The Book of the Garand' which was written in 1948 refers to 'WD' (War Department) specified steel number codes.
He states the barrels were 4150 Modified, early receivers 3115, early bolts 3312, with a standardization after 7-42 to 8620 for both receivers and bolts.
This is interesting because the M1 started production in 1937.
Winchester made half a million or so Garand rifles under contract so at that time they were working with 4150, or a version of it close to the commercial grade.
The war department found 4150 necessary for .30-06 barrels of the period, at least those used in semi-automatic rifles. It is still used today by various barrel makers.
I wonder what Winchester settled on for the magnum calibers in their sporting rifles.
