Actually, depending on period of manufacture you are wrong, or not completely right, on both counts.

In 1974 I attended the S&W Armorers School as a police armorer. I was there for automatics, and since they did not teach automatics in the armorers school at that time was assigned to the automatic section directly under the automatic foreman. To learn the guns spent two weeks doing range repairs of guns that didn't pass the test firing. (Just background)

While there I asked a very specific question about just this subject which was basically "Between .38 Special guns and .357 Magnum guns is there any difference in either material or heat treatment?" The answer was NO, they are identical. Think about it, why would you want to stock two different alloys which require different heat treatment? This increases costs beyond the savings from one alloy possibly being somewhat less expensive than the other, and the risk of mixing up parts of different materials? It is less expensive from a production standpoint to use the same material and heat treatment for all guns and have some much stronger than necessary.

On the second point, the only difference between the normal KT frame for the 14, 15, 16, etc, and the frame which is specific to the 19 is a very slight difference in the contour of the front of the frame to match properly with the extractor shroud. It isn't thicker, wider, or different in any dimension except for this point, and this is purely cosmetic. Both the 15 and 19 frames, at least at the time the question was asked had identical material, heat treatment, and for anything affecting strength were of identical dimensions.

Is re-chambering a model 10, 15, 14, etc. from .38 Spl. to .357 Magnum a good idea? Not really, for several reasons, but strength isn't one of them.

Believe me or not, I really don't care, but don't argue I am wrong unless you have a better source of information than "It just sounds right to me".