The 1954 Rules of Proof “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula:
3 tons = 8,960 psi = 618 BAR
3 1/4 tons = 9,800 psi = 676 BAR

Major Sir Gerald Burrard, The Modern Shotgun, Volume II, “The Cartridge”, 1955 3rd Revised Edition listed standard service load pressures:
20g 2 1/2” 3/4 oz. 2 1/4 Dr. Eq. - 9,632 psi
20g 2 3/4” 7/8 oz. 2 1/3 Dr. Eq. - 9,968 psi