I guess about as close as we have to the early pressures are those mentioned from the early 20's. These were recorded for a 3DE load with 1oz shot. As noted older loads used up to 3DE of powder with the 1oz of shot. The 3 dram measure of DuPont Bulk load certainly did not fall into what we refer to today as "Low Pressure"
In an srticle on Shotgun powders in the american Rifleman back sometime in the 1950's they stated that Infallible was the same powder as Unique. 26 gtains of Unique with 1oz shot is not a particular low pressure load even in paper cases with card & filler wads & can only really assume that neither was the same load using Infallible.
Another point is that when some of the old makers stated "Bored For Smokeless" this often refered to the size of the hole down the bbls. Many of the older gunes were "Bored" for the use of the all brass shelles with black powder & thus had oversize bores. Re-loading with smokeless was generally discouraged at that point of time. Thus a person who intended using smokeless was expected to be buying factory loads put up in paper cases with standard wadding so the barrels were bored nearer to standard gauge size.
I personally still prefer for my sound damascus guns to shoot relatively fast powders with loads in the 7-8K psi range than to court disaster with those Super-Low pressures in the 5K range acheived by underloading the slow powders. In my Lefevers by comparsion to the loads listed in their catalogs even the 8k loads are quite mild.
