liverwort:
The 1928 edition of “Smokeless Shotgun Powders” by Wallace Coxe, ballistic engineer of the Burnside Laboratory of the E.I. du Pont de Nemours & Co. reported 3 1/2 Dram Eq. 1 1/4 oz. loads:
NOTE: pressures were measured by crushers (LUP) and modern transducer measurement pressures would be 10 – 14% higher
DuPont Bulk smokeless powder - 11,700 psi
Schultze Bulk smokeless powder - 11,800 psi
28 grains of Ballistite Dense smokeless powder - 12,600 psi
(Note all 3 are greater than the SAAMI 12g 2 3/4” recommended maximum pressure of 11,500 psi)
40 grains of DuPont Oval Progressive Burning powder used in the Super-X loads - 9,400 psi

There is a Hunter Arms Co. Pressure Curve dated June 10, 1929, from the McCracken Research Library, Buffalo Bill Center of the West (found by Gary Rennles) very likely from DuPont
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/107
12g 3” ‘Record’ 1 3/8 oz. shot presumed 1275 – 1295 fps = 13,250 psi
12g 2 3/4” Super-X ‘Field’ 1 1/4 oz. shot presumed 1330 fps = 10,750 psi
12g 2 3/4” 1 1/4 oz. 28 gr. (Dense) Ballistite = 12,900 psi
12g 2 3/4” 1 1/4 oz. 3 1/2 Drams DuPont Bulk = 10,000 psi
+ 10-14%