You forgot to mention that on many, if not most, postwar guns an N was also stamped on the rear face of the cylinder if the nickel is factory. All of those in my collection have this feature, but I'll grant that none of them are from the 1950s. All my S&W revolvers from that decade are blued.IIRC pre-war blue guns had a "B" in front of the serial number on the bottom of the barrel and nickel guns had nothing. Reverse for early post war pre-model guns. The nickel guns had an "N" and blue guns had nothing. Post-model nickel guns had the "N" on the side of the frame under the grips.
As always with S&W this is not all inclusive but it is what I have observed.
You forgot to mention that on many, if not most, postwar guns an N was also stamped on the rear face of the cylinder if the nickel is factory. All of those in my collection have this feature, but I'll grant that none of them are from the 1950s. All my S&W revolvers from that decade are blued.
My 1950's guns also have an N stamped on the rear face of the cylinder under the ejector star, like this...
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But that number under the extractor is not typical for a production gun in that number range. I'm assuming it is there on that gun because it is engraved.