In this case, .38 Special +P is an official SAAMI industry specification for ammo. Where .38 Special ammo is loaded to a maximum peak pressure of 17,000 PSI... .38 Special+P ammo is loaded a wee bit hotter to a maximum allowed peak pressure of 18,500 PSI.
This is as set/agreed upon by SAAMI, which is recognized by the firearms manufacturers.
The thing that many folks don't understand about the magical "+p" designation is that it is a true industry standard and it is being misused or abused when it is casually tossed around. For the record, SAAMI has used this designation with only three handgun cartridges-- .38 Special, 9mm Luger and .45 ACP. Used elsewhere...is used either in error or recklessly. (a special case exists for .38 Super)
Also worth noting is that SAAMI doesn't use or recognize the "+p+" designation, even though the occasional ammunition manufacturer has used it. What's important to remember is that if it is not made to a recognized standard it certainly could be made to any standard -- or NO standard. That makes it an unknown quantity and that means nobody can be sure if it's safe or proper for your firearm.
Smith & Wesson revolvers since they began to assign them model numbers are safely rated for +P ammunition. (I think? Correction please?)
YOUR revolver bearing that 400xxx range is not rated for use with .38 Special +P ammunition.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.