I've always been suspicious of these Model "changes" that happened exactly on a major number such as 4000, etc.
I can't believe that S&W, when they realized that they had a turkey in production, waited for a nice even number to make the change or made exactly 4000 guns and then realized they had a bummer on their hands. I don't think things were quite that neat and orderly.
It is my understanding that most of the serial ranges published are just an estimation based on observations rather than actual records.

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Dean
SWCA #680 SWHF #446