Seriously have to question this... if the SAAMI standard is to use the same proof pressure for both 38 special and 38 special +p, why are companies like Taurus and its 856 revolver, Armscor and its 38 sepcial revolver, both only listed as STANDARD 38 special only.
Ok, here's the serious answer. There is a difference between the proof pressure limits and the WORKING pressure limits.
The proof load limit is based off the general pressure of the cartridge, increased by a specific amount, to be an intentional overload, that the gun must survive ONCE (or sometimes two or three rounds) without damage.
The proof limit is the same for the .38 Special and the .38 Special +p because they are the same cartridge. The +p is just a heavier (higher pressure) loading than standard.
The SAAMI pressure standards commonly talked about are the WORKING PRESSURE limits, for commercially produced guns and ammo. SAAMI is voluntary and no one is forced to comply if they don't want to.
Each maker can rate their guns any way they want, in the case of .38 specials, the gun survives proof loads. The gun WILL survive +p loads, BUT those loads put more stress on the gun and odds are it will wear out sooner, and that is something the maker wants to avoid, and can easily do so by not "rating" a specific gun for +p.
Informally they will often tell you the gun will take a moderate amount of +p shooting without trouble, but officially its not rated for +p, so if something fails, because you shot +p, that's on you, not them.
Colt originally didn't rate their Agent/Cobra snubnoses for +p, then later they sort of did. Sort of because while they said +p was ok they also wanted the guns sent back to the factory to be checked and inspected after every 1,000 rnds of +p ammo fired.
SO, whether or not a specific gun is mechanically capable of handling +p is not necessarily the deciding factor in a company's decision to rate it for continuous +p use, or not.