If the markings are faint but the blue is strong, I'd suspect that it has been polished and refinished already. If the blue is mostly gone and the markings are somewhat rubbed away, that's a lot of, but at least honest, wear. Restoring the markings is possible, I suppose, although S&W no longer takes on that kind of work. Afterwards you would have to reblue the gun, so following that expense, what you would end up with is a "restored" gun that would possibly not be worth what you have invested in it....