John is correct, which is not to say the guns were not proved.
Parker Brothers 1893 Catalogue
“Our guns are bored on the latest improved system for shooting Nitros, or Smokeless Powder, and all our guns are tested with some one of the most approved makes, and a tag accompanies each gun, giving the results of such a (pattern) test.”
(No records exist regarding the specific proof load in that time period.)
“A Trip Thru Parker Bros.”, 1923. Courtesy of Jeff Kuss
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 is 10,500 psi. Definitive proof used 7.53 Drams (no doubt) black powder and 2 oz. shot with a pressure of 15,900 psi. The pressure was measured using LUP and modern transducer values would be 10-14% higher, or more than 17,500 psi.
LTC Calvin Goddard writing in “Army Ordnance” in 1934, reported that Parker followed the SAAMI standards then in force: 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.