The question was about .38 Special +P, NOT +P+.
There are no SAAMI standards for +P+ since it was made for LE use only and never sold to the public or released into the commercial system.
I have seen boxes of .38 Special +P+ marked to be used only in revolvers chambered for the .357 Magnum.
.38 Special +P (NOTE SINGLE +) runs 20,000 psi, compared with 17,000 psi for standard .38 Special, and 14,500 for .38 S&W (and 35,000 for the .357 Magnum).
(The .38 Special +P+ was developed for police use as a PR measure. It provided near .357 Magnum power, without using the term "Magnum" which had been the cause of protests against police "brutality".)
Jim