Just to clarify (or perhaps muddy the waters). There is no industry standard for +P+. SAAMI establishes standards and, for some calibres, a second higher+P standard.(And SAAMI seems to be lowering those "standard" pressure ceilings every decade or so for handgun calibres). This is not to say that the makers of +P+ don't have their own standard. NE450No2 mentions a 23,500 psi ceiling for those treasury loads. Once upon a time +P 38 spl had a ceiling of 22,500CUP now it is 18,500 psi .......less than the "standard" ceiling of pre SAAMI days. Once upon a time there was the 38-44,,, and noone seems to KNOW what the pressure ceiling was on those factory loads but based upon velocities some conjecture that it was 30,000cup.......then we had 38 special Hi-speed.....which was advertized as a bit slower than the original 38-44 and might just barely have been achievible at the 23,500psi mentioned earlier(and those cartridges only had the warning that they were for "modern arms in good condition"). Long ago Kent Bellah had the temerity to actually publish his personal role in a public relations slight of hand with the Texas Dept. Public Safety(Highway Patrol) it seems an incident occurred that brought their use of .357 magnum ammo under scrutiny and negative legal pressure. They commissioned Bellah and others to solve their problem and so a .38 Special load was put into service which developed just under 30k CUP. Essentially it matched our current .357 factory loads( a bit behind the .357 mag of its era) strained nothing in the Patrolmen's.357 revolvers and was deemed safe (though not recommended) should it wander into a Chiefs Special....And the Officers were "only shooting .38 Specials" in their .357 revolvers.
What is +P+? It is hard to say, but it probably meens a 38 special loading that produces somewhere between 20kand 30K psi. A heavy loading but not likely to destroy any .38 Special-They all should have passed a higher proof loading.