I’m pretty sure Dale has it nailed down.

The British post-war proofs from Birmingham were mostly placed on the other side, or under the barrel on shortened guns, so they may be there.

Assuming no prefix, the serial makes it most likely a former British Service model shipped in early 1941 to the British Purchasing Commission in New York.

There would be a plugged lanyard hole in the butt and the serial off-center. If the serial is centered and no hole, it would have been a pre-war commercial M&P, considerably less likely.

Checking whether the cylinder has been converted from .38 S&W to Special also offers a clue.