From the SAAMI Technical Data Sheet: Maximum Average Pressure--is the recommended maximum pressure for loading commercial sporting ammunition. Slightly different words, but it's the same thing the CIP refers to as their standard service pressure.

Found any of those pre-WWI Spanish guns yet, Kyrie? Photos maybe? To include details? I find it interesting that Spain did not even adopt obligatory proof until 1924. And that before things became more "standardized", Spanish makers were in the habit of copying proofmarks from other nations. Might explain why not many of those guns made it out of the country.