welcome01 to the forums from the Wiregrass! Would you post a picture of the cylinder face and make sure the chambers are in focus so we can see into the chambers. It appears that British Service Revolver has either had the cylinder reamed for .38 Special or the cylinder was replaced. If the SN on the cylinder matches the barrel and butt, it is all original and the cylinder was likely reamed to accept .38 Special.

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Guy
SWHF #474 SWCA LM#2629