Totally agree with Peter. The 'Standard' UKMilitary Proof marking. Observed by all Armourers. Peter underestimates his statement of seeing this mark a Zillion times.
It is more like Twenty Zillion times for Us! As it was part of the procedure in ANY inspection, to view Proof Marks. On any Barrel, Reciever & Bolt/ Breech Block. On
ALL in Service Small Arms. This was stated in Inspection Standards. As a 'Safety' procedure, to ensure. That High pressure Components had Been Proof Tested.
Military Proof is of a much higher standard than Civilian. In that Civy Proof generaly involves one Proof round to be fired. Military Proof, TWO rounds are fired.
One Dry (Standard Proof) & one 'Wet' (Oiled) The reason oil was used, was that it increased pressure on the pressure bearing parts, during Proofing.
Basicaly, a Proof round is one & a half times MORE Breech Pressure than a 'Standard' Round. The thinking behind this standard is: If it will withstand the pressure of a Proof round. it will easily handle any off the shelf available Ammo. Does not quite take into account an incorrectly hand reloaded private/Civilian Round situation!
But you will understand, the need for a baseline safety starting point!