Welcome to the forum!

In picture #5 which shows the rear sight, directly under the sight are the words "Nitro Proved" followed by crossed sword proofmarks. These look like an X, which appears to have tiny letters stamped around it. There is another similar mark just after the first one, and another again on the action.
Can you make out what the letters are and also tell us in what position the letters are, in relation to the X, ie to the left, right, above or below the X?
Thanks