"There is no difference in the steel used or heat treating done when the change over to longer cartridges came about."
Uhm... well, no.
Virtually all gun manufacturer's engaged in continuous upgrades to both steel compositions and heat treating methods as the 20th century progressed.
As better steel blends became available from the manufacturers, older blends became obsolete.
Heat treating also changed due to the new steel blends; it also changed due to increasingly better heat treating equipment, including pyrometers, as well as better laboratory testing techniques.
If, however, you're saying that Savage didn't change its steel blends and heat treating in 1954-56 when they started introducing .308-based cartridges, that's more than likely the case.