I agree with checking with S&W. But +P is nothing more than the "warm loads" handloaders made up for years, and I think it would take a lot of them to harm a post-war K-38 (or any other fairly modern K frame).
+P+ is another story. I think it is what Fuff is thinking about. It is definitely in the .357 Magnum pressure category, although no figures have ever been published as it was for LE only. The boxes are marked to be used only in revolvers chambered for .357 Magnum.
Fuff is right that it was a politically correct round. If the cops used .357, they were accused of blowing crooks into little pieces, wiping out buildings, causing mile wide craters, etc. With the +P+ .38, the police PR guy could say that they weren't using that awful .357, only the old .38, and the pro-crime folks couldn't get a handle on that.
Jim