These stamps are British proof marks that show that this weapon was sold on the commercial UK market - if you post a good shot of the crossed sceptres we can tell you when this happened.Bob
these are some of the marks on the 445 also there are marks all over it to small for me to take a pix right now...NORM
The .760" marking shows the length of the chamber, and '6 tons per [square sign' is the standard working pressure of the .455 Webley cartridge.
There SHOULD also be other stamps, such as Crown over BNP, or BP.
These stamps are all indicative of a service weapon sold off into the public domain by commerce enterprise - it is illegal in the UK to sell any weapon without a valid proof.