Some may think this heresy, but there is the strong possibility that this is a gun that was made in Japan and prior to WW2.
I won't go into all my reasoning, at this time, but will say that the long number that you cite along with the bracket frame , that looks distinctly like Westley Richards, may give its origin away.
It is possible that the gun was sold as used, in Japan, and brought back to England.
Best,
John

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