I think you're looking for something that was not published or documented because it was not needed or even thought of at the time Jon.
The need for 'service pressure' standards arose with the development of smokeless powder.
With black, you charge by weight and what you get is what you get. Thus, the '3 dram' load.
Jones, I have no exact dates at hand right now, but I'm sure you'll find that ballisticians and proof houses had pretty accurate means of measuring peak pressures before smokeless powder came into common use. In fact, they were measuring pressures at various points along the length of gun barrels, and plotting pressure curves. This information helped gunmakers design and build barrels with adequate wall thickness to withstand firing without failure, and with a built in safety margin without excess weight.. But I'll bet you knew this because we have discussed ballistics, and I know you are hardly a novice in these matters.
If I get time later, I'll try to
copy and paste some history of pressure measurement so I can pretend to be a shotgun expert... or I can just smile about raising someone's blood pressure to aneurysm popping level.
