Likely manufactured in the late 1920's with that mushroom extractor rod knob. When the Great Depression hit in 1929 Smith was stuck with a lot of guns they could not sell. . .some were finished and some were not. My guess is that your gun was manufactured in 1927-1928 and held in work-in-process inventory for several years. The grips were likely added sometime in the 1930's when the gun finally sold and shipped. Not uncommon to see a late 1920's gun with correct 1930's grips.

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Dr. B