Factory pearl stocks before 1898 did not have medallions, but were the same thickness as wood or hard rubber stocks of the time. Measuring a set of hard rubber stocks below the screw where they are the thickest and compare to your set will tell you if they are likely originally from the factory or not. It is possible that someone could get hold of a set of original 38 DA pearls and put them on an I frame from a later period. Anything after 1898 should always have medallions. Chronology is below.
Originally, Smith & Wesson pearl grips were made without any medallions, and no pearl grips at the Columbian Exposition in 1893 had them. The company (S&W) discovered that rather than purchasing guns with pearl grips from Smith & Wesson, distributors bought guns with standard factory grips and installed cheaper third-party pearl grips. To protect its reputation, and profit, the company started placing gold-plated Smith & Wesson trademark medallions on all factory pearl grips in 1898. At the same time, the factory also replaced the pearl grips on its 1893 display guns with new medallion grips.
Robinson was one of the suppliers of the pearl blanks to S&W and had been making pearl stocks as well as ivory going back to about 1868. The factory stopped purchasing MOP blanks around 1910 and worked off inventory until they were gone. Around the late 1930s, S&W ran out of inventory pearl stocks, stating they could no longer rely on obtaining quality pearl for stocks.
