This? The Parker Bros. Overload Proved stamp first appeared about 1925
A Parker Service and Proof Load table was published in the 1930s and reproduced in the
The Parker Story p. 515.
12g 2 3/4” shell service pressure was listed as 10,500 psi.
Definitive proof used 7.53 Drams Black Powder and 2 oz. shot with a pressure of 15,900 psi. The pressure was no doubt measured using LUP and modern transducer values would be 10-14% higher, or more than 17,500 psi.
LTC Calvin Goddard reported the same numbers in “Army Ordnance”, 1934. He wrote that Parker followed the SAAMI standards of that period: 13,700 psi proof, 9500 psi service for 2 5/8” chamber; 15,900 psi proof, 10,500 psi service for 2 3/4” chamber (by LUP) + 10-14% for modern transducer measurement.