The MADE IN U.S.A. frame stamp began around mid-1922, and it is indeed a commonly used indicator of age. Another is the extractor rod knob. The "mushroom" shaped knob was changed to the barrel-shaped knurled knob around 1928. The S&W logo was for a long time quite small and stamped on the left side, but went to a larger size on the sideplate around 1938. No logo was used during the WWI period. In some cases, the grip style can be an aid in determining age, assuming the grips are original (they often are not). But the best way to come up with an approximate date is from the serial number. You can always get fairly close to the shipping date if you know the SN.