A Hunter Arms Co. Pressure Curve dated 6-10-1929 is in the records of the McCracken Research Library, Buffalo Bill Center of the West
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/107

Proof pressures were measured by crushers (LUP - Lead Units Pressure) and modern piezoelectric transducer measurements would be 10 – 14% higher

12g 3” = 17,250 psi or about 19,000, which is about the modern SAAMI proof load standard
12g 2 3/4” = 14,200 psi or about 16,000 psi

The modern SAAMI max. 12g service pressure is 11,500 psi, which Western's Super-X 1 1/4 oz. "Field" equaled.
The 3” Super-X “Record” with 1 3/8 oz. was about 13,000 psi.

It is of course reasonable to protect the wood with lower recoil loads, or have the head of the stock glasbedded.

There is more information in the FAQs "Shotshells and Shooting"