With the 1933 National Industrial Recovery Act the American Iron and Steel Institute began publishing Steel Products Manuals with the Steel Code Tables. The Society of Automotive Engineers also published a SAE Handbook with a similar numbering system, and in the early 40s the tables became the SAE-AISI Code Designations.

I've got a little about vintage rifle barrels starting on p. 35 here
https://docs.google.com/document/d/1dnRLZgcuHfx7uFOHvHCUGnGFiLiset-DTTEK8OtPYVA/edit

https://sites.google.com/a/damascusknowledge.com/www/home