Art Doc and I have had some differing opinions about this matter. He posted above that Model-marked S&W's have no changes from pre Model-marked guns.
In 1960, famed holster maker Chic Gaylord noted in his, Handgunner's Guide that a S&W factory official had assured him that the Model 10-6 and presumably other K-frame .38 Specials were being made of a new and stronger steel and that the guns were stronger!
Gaylord was an advocate of Colt's Official Police, which was larger and had better metallurgy than S&W guns, so he felt this was important new information and included it in his book!
This jives perfectly with the factory's admonition to fire Plus P in STEEL framed guns with model markings, post 1958.
Now, the METHOD of measuring velocity has changed and this has led some shortsighted and uninformed persons to think the ammo was weakened. The reason for different figures published is that ammo is now fired in four-inch vented barrels, not in longer solid breeched pressure guns. So, velocities look slower on paper.
And what do we mean by Plus P? Buffalo Bore and Underwood offer significantly hotter Plus P than do most makers. I think it's folly to shoot those hot rounds in guns made before cylinders were heat treated after very late 1919. Actually, I'm not comfortable firing it in any S&W .38 made prior to 1958 when model markings appeared. And I'd use it sparingly when needed in modern guns. They'll last longer without repairs that way.