These are the earliest pressure numbers I've found reported as psi (measured by LUP) rather than tons
1892. Service charge 1 1/4 oz. 3 1/4 Dr. Eq.
https://books.google.com/books?id=in...AJ&pg=PA296&dq
The Overland Monthly, Oct. 1895 “Smokeless Powder For Shotguns” with higher numbers
http://books.google.com/books?id=Wv0...J&pg=PA446&lpg
p. 453
3 1/4 Dram Eq./40 grains DuPont Bulk Smokeless 1 1/8 oz. (1255 fps) = 7440 psi
3 1/4 Dram Eq./44 grains “E.C.” Bulk Smokeless 1 1/8 oz. = 7584 psi
For comparison, a DuPont loading manual from the
1960s listed DuPont Bulk:
3 1/2 Dr. Eq. with 1 1/8 oz. shot at 10,000 psi
3 1/5 (3.20) Dr. Eq. with 1 1/4 oz. shot at 9,900 psi
(Probably measured by LUP)