In W W Greeners 9th Edition (1910) he pictures the "Crusher Gauge"; pressure gun using lead or copper crushers for measuring peak pressures. Pressures may well have been accurately measured for a little longer than most realize. they were however likely known mostly to gun & ammo manufactures & not generally published. In the 1913 catalog there are loads listed for the various gauges which no doubt ran into the 9k or above pressure level with no cautions about which type of their bbls they were to be used in. For instance in 12ga they show up to 28grains of "Infallible" powder under 1¼oz shot. According to an old article from the "American Rifleman" Unique & Infallible were identical. Even with card & felt wadding this is not a particularly mild load. I think pressures at the turn of the century may well have run a little higher than we are often led to believe. The thing to remember though is these guns are now a century± old. I lack some 30+ years of being a century old, but can't do what I could a mere half century ago. Treat them kind & they will just keep on keeping on.
Personally as my use of these guns is afield, their use requires fewer shots than going to the target course, I prefer to go no lower than aboout 7K in 12ga & 8K in the smaller bores. Lower pressures are simply not always reliable in colder weather.
