As someone who has been part of the Automotive Supply chain for well over 30 years I have to say that I would be absolutely SHOCKED if this statement were true. Because I can assure you that if two parts are visually identical they will be mixed together at some point in the manufacturing history. Meaning at some point material intended for a model 19 will use material for a model 15. With the Liability issues involved the ONLY way to insure that a model 15 cylinder bored for a 357 Magnum by mistake and installed in a model 19 doesn't blow up is to insure that every single model 15 frame and cylinder are identical in the Mechanical Properties and Heat Treatment. If S&W were to attempt to keep two different grades at some point we would have seen a recall for model 19's with the wrong cylinder material or heat treat. Because I can assure you that people on the production line would make this kind of mistake. Heck, I've seen pictures of model 15 cylinders posted on this forum that were bored for 357 Magnum, so it's pretty obvious that this has happened. I also expect that there are folks who have purchased model 19's and found they wouldn't chamber a 357 Magnum, something that could easily be corrected under warranty.

Now, for K-38's made prior to the introduction of the Combat magnum, I would expect that those did indeed use a "lower grade" steel and heat treat. They may have also tried using a dual material approach during the early production of the pre-15 and 19 but I would expect that one or two incidents where parts got mixed taught S&W very early that this just isn't a "safe" approach and that the difference in cost between the two grades was either nothing at all or very minimal. End result is that any responsible manufacture would only use the higher grade material for both model lines.