Based on your description and the poor photo, I’d say it’s a genuine S&W British Service Revolver that was converted to .38 Special.

It was a common practice after the war when many of these were sold commercially. British law required that the guns be proof tested and so marked before they could be sold, but those markings are not English proofs.

No idea where the conversion was done, but I’d steer clear of it. The .38 S&W cartridge is fatter and shorter than a .38 Special and most of the conversion consisted of simply reaming the chambers, which typically results in bulged brass.

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