I zoomed in on the right side of the barrel and the “symbols” next to “NP” appear to be British proofs. The NP and associated marks would signify that the gun was nitro proved. Try to get a photo of the proof marks.
The gun doesn’t appear to have the “Not English Made” stamp. My guess is it went to Great Britain under Lend Lease, but wasn’t sold commercially after the war. I don’t know why it has the S&W trademark, but I’d pull the sideplate and check the number on the inside.
Do your barrel markings look like this?