"The BNP 3-1/2 Tons is a British military proof mark."

Actually, this is a British commercial proof mark, when it was sold into the commercial market after its release from military service. 3 1/2 tons was the proof load pressure, and 0.767" is the length of the .38 S & W case.

The best way to tell if it has been re-chambered to .38 Special is to see if one chambers; if so, it has been converted, since the S & W case is shorter.

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Alan
SWCA LM 2023, SWHF 220