Cassady, the Birmingham date mark shows that the gun you are discussing was proofed in 1957.

Brian, according to Brown's "London Gunmakers" your gun was made between 1926 and 1929 (1925 - 54,960; 1926 - 55,166; 1927 - 55,603; 1928 - 55,754; 1929 - 56,209).

Regards - Ian Forrester