When S&W went from named models to numbered models, sometime around 1957, the .44 Magnum for example, became the Model 29. The Combat Magnum became the Model 19, etc, etc. After that time, whenever S&W made a design change to a model, they added a dash and a one, then a two and so on as other changes occurred.
I don't have my Standard Catalogue of Smith and Wesson handy, but it has all the info on what those changes were. Some of the changes that between a -2 and -3, etc of certain models are very minor. One small thing was changed, but it was still a change so a -4 went to a dash -5, or whatever the case may have been.
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