You got it.

John: The 1954 British Rules of Proof refers to that number as "Highest Mean Service Pressure"

The terminology and numbers changed when Great Britain adopted the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives transducer (no longer crusher) standards in 1989
The CIP transducer “Maximal Statistical Individual Pressure” is 850 BAR = 12,328 psi for a “Maximal Average (Service) Pressure” of 740 BAR = 10,733 psi.

Someone must have really liked a GHE Parker Bros. to spring for a W.R. sleeve job!! That must have cost a lot more than any 12-gauge GHE was worth at the time.

My thoughts exactly. And a restock to boot. That had to cost more than any 12 gauge GHE is worth today.