R. Reynolds
You are correct that under the 1954 Rules of Proof the “Highest Mean Service Pressure” equivalent transducer values as converted from LUP by Burrard’s formula for 3 tons = 8,960 psi = 618 BAR

Scroll down about 3/4 here for the short version of the confusing evolution of British Proof and Service pressures
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/edit

https://sites.google.com/a/damascusknowledge.com/www/home