Steve:
Your Smith & Wesson started off as model K-22 Combat masterpiece. 1957 was the year when the first stamped model 18 was produced. The model 18-1 came out in early 1959 was change extractor rod, right hand to left hand thread.
A K serial prefix was early postwar product and has 6 grove backstrap/forestrap.
Your serial number puts it in 1958 production date.
Hope this information helps. It came from the book Standard Catalog of Smith & Wesson, Second Edition. There is a newer version of this book on the market as well.
Hope this information helps.
You can contact Smith & Wesson at 1-800-331-0852.
Regards,
George
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