Any Smith with an actual model number is fine for +P.
Not quite. According to S&W, any K or N Frame revolver with a steel frame and model number are fine with +P ammo (L Frames in .357 Magnum can safely fire .38 +P ammo too, but they were never made in .38 only or without model numbers). Older J-Frames (made prior to 1999 IIRC) are not rated for +P ammunition nor is the Model 12 K Frame because it has an aluminum alloy frame. Personally, I will still use +P ammo in a steel J Frame made prior to the cutoff, but not a steady diet of it. I would not, however, use +P ammunition in an aluminum-frame gun not specifically rated for it.