I have never read anything documented from Smith & Wesson regarding N-frames having specific heat treatments for different calibers.

There is no doubt that cylinders receive heat treatment and I believe that is across the board.

The determining factor in strength has a great deal more to do with the thickness of the cylinder walls than any other factor. I am the owner of a Ruger Redhawk in .357 that was made for a very short time in the eighties, I have never seen a cylinder with more metal thickness than this revolver displays’ and I have still never exceeded top posted loads for this caliber.

I have complete faith that Smith & Wesson designs their N-frames to comfortable exceed any SAAMI standards developed for the respective calibers.