My 640 no-dash was built in 1990 according to S&W. It has the CEN prefix and a number above 4000, and is etched inside the grip frame (bottom strap) "Tested For +P+".

It's a very fine little revolver. I see no need for one in .357 Magnum, but whatever suits you is yours to decide. It seems to me you have to really like recoil, flash and blast to shoot .357 stuff in a snubby, even an all-steel one.

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Oh well, what the hell.