I have read on several occasions that Smith & Wesson may have started heat treatment on some of their firearms as early as the 1920 era, as I am under no circumstance a historian of Smith & Wesson there may well be some on this site that could confirm the facts of that issue.

The reason for heat treatment of Stainless steel metals is as follows:

Stainless steels are often heat treated; the nature of this treatment depends on the type of stainless steel and the reason for the treatment. These treatments, which include annealing, hardening and stress relieving, restore desirable properties such as corrosion resistance and ductility to metal altered by prior fabrication operations or produce hard structures able to withstand high stresses or abrasion in service.