21's are almost impossible to date based on their serial numbers other than the very early guns. This is because the big-W made a bunch of action frames all at once in the beginning of the production run. They assigned & stamped serial numbers to them at the time and basically threw them in a bin waiting for their turn at the bench. Over the years when a craftsman needed to start a gun he would just grab an action without regard to serial number continuity.
The only definitive way to date or grade a model 21 is to obtain a letter from the Cody museum and even then they might not have the records or will have only partial information about it.
The best thing you can do is study up on 21's look at a bunch of them and use your best judgement. Keep in mind that over the years there has been many 21's made up from parts and represented as the real deal. 28ga's are the ones you really have to keep an eye on in this regard.
