I know it doesn’t say “7.62”, but for general interest my 2023 AT bought this year from Eurooptic has this Brit stamp:
View attachment 8431547
I don’t pretend to know what it means…found some shotgun proof mark references but they relate to using steel shot or not.
Just means Proofed to CIP Nitro standards. By Birmingham Proof house in 2023.
HKSniper11B - You might want to extend your search, the barrels your looking for might not have a crown on, signifying Birmingham Proof house, entirely likely you could London Proof house markings like below: