English Proof Mark on a MK I .455

[Sunray]

"...a nitro proof..." The BNP stamps? Those are required by English law on any milsurp sold through there. Nothing to do with the military or refurbishing.
Should look like this.
BNP under a king's crown = British Nitro Proof, .455":.760" = the case length of 455 Webley Revolver cartridge, 5 TONS = Pressure of the proofing. (5's can look like 6's)
A Mk I can be over 100 years old but the Brits would still insist on stamping it.