The principal difference isn't in the frame, but the cylinder. Magnum cylinders were made from a different alloy steel, and heat treated differently.
You have to shoot a lot of Plus P ammunition to see any effect, but what eventually happens is cylinder back & forth play (as opposed to rotational play) that is caused by the cylinder battering the end of the soft yoke barrel, on which the cylinder revolves. This is not hard to correct, but S&W saw no reason they should have to fix it under warrantee. They say, "absolutely no" on pre-1957 revolvers, partly because they don't always have parts to fix them.