Per "U.S. Cartridges and Their Handguns" Charles R. Suydam 1979, page 172.

"First chambered in the First Model Hand Ejector in 1899, the .38 Smith & Wesson Special cartridge was first made by UMC early in 1899; cartridges were sent to Smith & Wesson for trial in May of that year. Original loads were of 18 grains of black powder, with an overall cartridge length of 1.570". In June, 1899, the powder charge was changed to 21.5 grains; bullet weight was presumably the 158 grain still used. The first smokeless loadings were made in September, 1899, probably 3.6 grains of Bullseye. In 1908 the overall length was reduced to 1.560"; when the present nominal length of 1.550" was adopted is not known."