"A student of Model 21s can look at a gun and make a decision on originality without a Cody letter. I have never sent for a Cody letter before buying a Model 21. I just know, after studying and screwing with these guns since 1955 and owning and buying them since 1965."

I disagree. I will cite two examples.
First is the above mentioned 20 gauge set offered to me as a 'Grand American'. While not a 'Grand American' it certainly looked for all the world like a 21-6 w/inlay except that the shade of color on the wood was just a little different than I was used to. The price offered was $20K about 6-7 years ago when 'Grand America' sets in 20 were selling for $30+K. As I said the inserted letter in the Winchester records said the serial # was a 28 gauge M21. Purchasing such a gun even at $20K or probably less if I decided to purchase would have been a waste of money if I could not prove original to another buyer. There were many providing upgrades to M21's and some did a good job. These guns are still out there.
Second example was just recently a person was offered a nice M21 ascribed to Nick Kusmit at a reasonable price for the Grade 6 engraving. Winchester records showed the gun left as unengraved. The engraving was very good an certainly looked like Nick's work. Personal contact with Nick showed that the gun was an aftermarket upgrade by Nick. Now that in itself is not a bad thing but without Nick's records there was no way to tell except that he did put his initials on the engraving but anyone could have engraved NK. The gun was purchased.
I certainly agree that many non factory upgrades can be spotted 10 feet away but there are some that are very very good and there are some refinished M21's that are very very good also and for standard grades the $$ bite was/is? not that bad but for engraved examples where the $$ is much higher, confirmation is the best bet.
Most prospective purchasers are not students of M21's and don't possess the neccessary acumen to make a decision based on experience. In that case a Cody letter is certainly justified.-Dick