I am guessing that S&W does not individually heat treat frames, and cylinders, barrels. It would be too costly, it does not make sense. I do believe that when the 19 was introduced they used treating to take the gun to the level for mild 357 use.

Car manufacturers used to use different sub frames for different models, that practice for the most part has been long gone. Most companies operate to make as much money as they can. They balance this with quality, and cutting costs. Thus the reason for MIM parts. It is not because MIM parts are better, or worse. It is because they can still offer quality gun at a price they can make money, and the customer can afford.

It comes always to follow the money.