According the SCS&W III, stamping was used from 1857 to about 1900, when pencilled numbers were written. In 1929, back to stamping until the late 1970's, when it was discontinued altogether. By then, dimensional uniformity from gun to gun was good enough that grips no longer needed to be fit to individual grip frames.
Generally, target stocks weren't numbered to their frames, while service and Magna stocks were.