You have a 4th Model Safety Hammerless, mfg. between 1898 and 1907. The S/N range was from 116003 to about 220000. I would guess your gun to be made in the 1900-1901 range.
186XXX shipped in 1905, so I think we can nail down when the revolver left the factory to 1906 - 1908. The antique cut off for the 38 Safety Hammerless is serial number 119,900. Anything after that is considered a modern firearm by the BATF.