Charles,
Older manuals often have data developed by pressure sign rather than by pressure measurement. Unfortunately, brass signs can change with brass brands, so you might get a different indication.
Of the manuals that do show pressure, a lot of them are still done using copper crushers. These instruments simply are not very consistent, as the spread in results in the table taken from a SAAMI test, below, shows. So what has happened in many instances is that as manufacturers gradually move over to the more consistent Piezo transducer systems of measurement, they discover the copper crusher measurements they made in the past actually run high when checked on the more modern apparatus.
That said, I also don't think people realize how much wiggle room there is in SAAMI pressures. The SAAMI MAP (maximum average pressure) is the number you normally see published. It is a number intended to be used as a manufacturing control, and is the average peak pressure for a single 10 round sample taken when the ammo is freshly loaded. SAAMI has a separate number, the MPSM (maximum probable sample mean) which is about 8% higher that is allowed for a 10 round sample average taken as the ammo lot ages. The maximum extreme individual load pressure that is allowed as an extreme variance is up close to proof pressure. Also, the MAP assumes a 4% standard deviation, which handloaders can beat.
So, the bottom line is a lot of judgment calls are involved in setting load limits. Western powders suggests lowering any maximum rifle load 10% and any maximum handgun cartridge load 15% as a starting point, then working up and watching for pressure signs in your particular gun.