Thanks again. Birmingham proof 1868-1875, with bore 13 = .710" - .728"
No re-proof.
Possibly Bro. Helsley knows the identity of 'JT' and 'JK'? Tube maker? Barrel finisher?
This list ends in 1857
My gun (4661) also has barrels by JT. There are no (old) marks on the barrel flats (which now carry the modern nitro reproof marks at 0.732 in). The new marks are small and unusually clear and well stamped. Any earlier mark would almost certainly be still visible.
I will see if I can find out who JT and JK were.