Drew;
When this chart was published in the American Rifleman (minus the Ballistite Curve) in the 1950's they stated the load was a 3 dram or equivalent charge with 1 oz shot. Transducers for measuring actual PSI was available by the 30's & it is highly possible that DuPont availed themselves of their use. We simply cannot say for a certainty that these pressures are LUP's.
The main thing though is the British pressures that Mr Goddard shows as being 4 to is the British load that we normally associate as a 3 ton/ 1 1/8 oz proof. All I am doing is citing the two sets of pressures & noting they do not match. If however Mr goddard did in fact cite actual PSI rather than LUP they match quite nicely. Transducers had been built by that point in time, so it is a possibility.
When the British worked out that LUP (tons) x 1.5 -.5 (ton) = PSI, they did so by firing loads through a pressure barrel equipped with both sensors so both measures were taken simultaneously. They did not state this to be an exact formula but that within the range of shotgun pressures that it was close enough for practical use. By this conversion a 3 Ton LUP converts to 4 Ton PSI. Mr goddard showed 4 ton. Close enough for me to believe he was showing actual PSI's. Nuff Said, unless someone can actually show me how mathematically that 3 LUP = 4 LUP.
