Only thing I can add, is it has Birmingham inspection marks, and Ezre Millward supplied a great many barrels to ordnance for the P'53. and many for its predecessors.
This one, (I see no date) may have been made as a smooth bore after the Mutiny, or possibly converted as Dave says.
Without digging out my books, I Think John Cook was the setter up.
There should be proofs on top left of the breech.