Toby, there are some marks on the barrels that are very deeply struck. The upper right mark is a pair of crossed swords with the letter B to the left, and O to the right. And below, it is some much deeper striking that is uninterprettable even under high magnification.
The new markings must be on a piece of inlaid steel. But the joint is very hard to find. Nicely done to be sure.
I bought this gun out of London from a reputable dealer in about 2005 or a bit earlier.
According to my notes the Birmingham Proof restarted stamping the letter "A" mark positioned to the left of the sword or sceptres hilts beginning in year 1999. Therefore if your marks have the letter "B" in this position it would have been marked in the year 2000.
