The reason you can't date a model 21 by the serial number is due to Winchester forging / machining a bunch of frames with serial numbers stamped on them in advance of anticipated sales. The sales of 21's were fairly slow and it took years to use the pre-made frames. When an order for a 21 would come in, the workman would just grab a frame and start the production of the gun without concern for continuity in serialization. The only way I know of to date a Model 21 is through the records at the Cody museum.