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Serial # range K555000-K605877 was circa 1964.
Prior to 1957, when Smith & Wesson changed from named models to model numbers, the Model 18 was referred to as the K-22 Combat Masterpiece. In general "Combat" referred to models with a 4" barrel. For example the K-22 Masterpiece, which became the Model 17 after 1957, has a 6" barrel and the K-22 Combat Masterpiece has a 4" barrel.
All Model 17 and Model 18 and their precursors are .22 caliber long rifle cartridge revolver.
Engineering/production changes to a model are designated by a - (dash) followed by the engineering/production change.
So prior to the Model 18-2 there was the 18 (i.e. no dash) and the 18-1.
The Model 18-2 was produced circa 1961 to 1967. The changes from the model 18-1 to the 18-2 included loss of the screw in the bottom of the frame just in front of the trigger guard (referred to as the trigger guard screw) and the cylinder stop was changed.
If your Model 18-2 has original stocks/grips there should be a serial number stamped on the inner surface of one of the stocks that matches the serial number of the revolver and the grips/stock should have "diamond grips" meaning there should be a small diamond pattern in the wood around the screw that holds the grips together.
"Diamond" grips were discontinued in 1968.
The barrel on a Model 18-2 is pinned. The pinned barrel was discontinued in 1982.
The Model 18 was discontinued at Model number 18-4 in 1985 although there was a brief reintroduction of it with Model 18-7 in 2009 so it's a bit confusing there are no Model 18-5 or 18-6 (?).
The 18-2 is built on a K-frame. Frame sizes for S&W revolvers from small to large: I-frame, improved I-frame, J-frame,
K-frame, L-frame, N-frame, and X-frame.
The Model 18 is finished in blue. There may be some rare examples of factory nickel finishes out there. There were a minimal # (rare) of stainless steel finish Model 18 from the 1970's.
The Model 18 should have an adjustable rear sight.