"on the side of the barrel Smith and Wesson .38 .767 BNP 3 1/2 tons"

This is a typical stamping found on the British .38/200 revolvers after they were proof tested for commercial sale after WW II. The case length of the .38 S & W (shorter and less powerful than the .38 Special) is .767". Most original examples had 5" barrels from cylinder face to muzzle.

Agree the serial number is too low for a pre-war gun; these SNs read with the barrel pointing right. With the V prefix guns they read with the barrel pointing left.

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Alan
SWCA LM 2023, SWHF 220