Standard .44 Special, .45 ACP and .45 Colt loadings are relatively low pressure, in the 16,000 CPU ballpark. As such S&W didn't see the need to heat treat the cylinders. If one wanted to handload extra-pressure loads they were on their own. For the record, Colt did just about the same thing.
.357 Magnum, .41 Magnum and .44 Magnum factory loads could be in the 40,000 + range, so they were the ones that got special alloy steel cylinders with double heat-treating.
As for frames. In both K and N sizes, Magnum and non-Magnum reolvers were made from the same frames, and often serial numbered in the same series.