All of the British proofed guns I have seen from this era had the Provisional proof, the Definitive proof, the View mark & bore size stamped in a line. If it is not a somehow goofed up 11 then it would not have been put there by the proof house. Why would the maker put another stamp in line with the proofs. "IF" it is a reversed J then it had to be made in reverse, you can't turn a steel stamp over & make it stamp backwards. You can turn it upside down, but not backwards.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra