To go back to your original question, there are a couple of things that you can do. If Cody has no records, you can check the link that Researcher kindly provided.
You can also go to the Griffin and Howe site and check to see if your serial number appears in their database. This would mean that it was purchased and sold at some point by A&F. If so, this may tell you more.
As the current owner, you can document as much about it as possible. If you do not currently have Schwing's book on the Model 21 I would recommend purchasing a copy and study the features and characteristics of your gun. Does your gun jive with what Schwing presents and illustrates? Again, write notes with dates and keep a file on your gun.
As to "authenticity" it is a Winchester Model 21. What a potential buyer will be interested in down the road is original condition. Or, what if any modifications have been done and by whom.
People like the letter. Whether it is from Cody, the PGCA, etc. Since Cody has no record, and if the other options show nothing, it will be important to have that self-generated file, to include the fact that Cody has no record. More importantly, be a good steward of the gun for as long as you wish to hold onto it. Personally, I do not feel that you will have any problem down the road selling an early production Model 21.
Finally and most importantly, enjoy it. Care for it and use it in your chosen activities. They were meant to be used.
Hope this helped.
