Brithunter--respectfully, I think you're mistaken. It looks to me like the mark on the barrel is the older one, not the newer (date-coded) one that began in July 1921, and was restarted in 1950. The one on this barrel is the private Birmingham mark that goes back to 1813. There's nothing I can see on the barrel to indicate that it was proofed (or re-proofed) in the 1950s.

I agree with 4seventy in his post above, that this barrel was originally proofed before 1904.

The pic you posted with the B/C is from 1952. But is it found on the original poster's rifle? If it is, then you're right, it indicates a proof in 1952, but it would be a reproof, not the original proof (which was pre-1904). However, old_LEESPEED didn't say that the rifle went back to England in the 1950s. That's the only way it would have got a 1952 proof. I'm guessing that the pic you posted is from someone else's rifle?