S&W has always used "hidden numbers" to aid law enforcement in cases where someone with criminal intent has removed the obvious number. On older revolvers it was in several places, notably on the back of the cylinder and on the front (toward the cylinder) of the extractor itself. Another place was on the bottom of the barrel under the extractor rod. Note that these were full serial numbers, not the assembly numbers commonly found on the crane and the frame under the crane. Nor were they for assembly purposes; S&W told LE officers that they were for the specific purpose of preserving a serial number if the normal one (on the butt) was obliterated or altered.