The issue of the whether to shoot +P 38 special ammo in a older S&W M&P is a myth. Most of it is the result of ammo makers downgrading there standard pressure 38 special ammo and relabeling what used to be standard pressure ammo to "+P". Before 1972 the standard pressure 38 special LRN round was running right a 850 fps. I was using this round in S&W M&Ps and early model 10s. If one is curious they can look on the internet and find several good articles by knowlegable sources confirming that the standard pressure 38 special round was running at 850 to 870 out of a 6 inch barrel. I never heard or scene a S&W K frame blowing up from using the standard pressure rounds in that time frame. Any revolver can wear out because of various reasons.
I have always maintain that a S&W 38 special steel revolver that is in good condition can shoot any round that is on the market right now.
Here is another article on the M&P 38 special hand ejector. It does reveal the first 38 special revolvers and the kind of ammo that was used.
Regards,
Howard
The First M&P - the Smith and Wesson Military and Police Hand Ejector Model of 1899