Those target stocks were made sometime between 1968 and 1982 (the color of the washers on the back could narrow it down further) and they were modified outside of the factory.
Someone did a nice job! I owned a set of smooth targets that were reshaped like that and they were very comfortable to shoot. S&W should have produced "modified Targets" for the square butt N and k frames, but they didn't. They did produce square but J frame target stocks like that.