As observed, the letter was written in 1932. Until the 1920s, the heaviest North American factory loaded 10 gauge shells offered were 1 1/4 ounces of shot with 4 1/4 Dr. Eq. of smokeless powder in a 2 7/8 inch case. Of course the unknowing could have substituted Dense for Bulk powder, more than doubling the pressures.
The Western Cartridge Co. Super-X load Super-Ten shell with 1 5/8 ounces of shot with 4 3/4 Dr. Eq. of Progressive Burning Smokeless Powder in a 2 7/8 inch case was introduced about 1926. The Western Super-X Magnum-Ten with 2 ounces of shot and 5 Dr. Eq. of Progressive Burning Smokeless Powder in a 3 1/2 inch case was introduced in 1932.
The only pressure data I've found is from the DuPont Brandywine Experimental Station data cited by Charles Askins in 1933; note this is
not for DuPont Oval but the earlier DuPont Bulk
10g 4 1/4 Dr. Eq. 1 3/8 oz. - 4.76 tons X 2240 = 10,662 psi + 10-14%
A Hunter Arms Co. Pressure Curve dated June 10, 1929 is in the McCracken Research Library, Buffalo Bill Center of the West and shows the 10g proof pressure but not 10g standard load pressures
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/107The 12g 3” ‘Record’
with DuPont Oval and 1 3/8 oz. shot (presumed 1275 – 1295 fps) is shown at 13,250 psi + 10-14% so the 10g 2 7/8" boomer load was likely similar.
