That stock is intended for a "Featherweight" frame, and the forend was designed for a set of manual extractor barrels utilizing the Curtis "push-button" forend fastener; don't remember the exact period in which Smith manual extractor guns were manufactured with the Curtis forend fastener, but 1913-1919 would cover the entire range. With Smith's "Featherweight" size frame, the primary size differences between the 12, 16, and 20-bore guns produced during this time frame would be in the size of the breech balls; but regardless of gauge all stocks were hand-fitted to frames, so one would be very fortunate to find a replacement stock that would fit another frame perfectly. As has been noted, the only way to know the gauge of the gun for which this stock was originally made is to request a research letter. As FYI, I believe you can also post these parts for sale on the LC Smith Forum (lcsca.org).
