How do we know when a new variety has been discovered? During the normal course of making coins or medallions, dies wear out and are replaced. Every effort is made to ensure that the replacement die is identical to the original so that pieces struck with either die cannot be differentiated without the help of a microscope or very powerful magnifying glass. No two dies can ever be exactly the same. Microscopic differences between the dies that can be seen on the struck pieces with the help of magnification are known as die fingerprints. Die fingerprints that cannot be identified with the naked eye generally do not result in a new variety being declared.
When a new die has an obvious difference from its predecessor, a new variety has been created. Die fingerprints that are discernable with the naked eye can also define a new variety. #65.1 is probably the best example of visible die fingerprint differences.
I had always suspected varieties of this sort and said so above the descriptions of the known obverse and reverse varieties on the $10 base page many years ago. Not wanting to renumber the encyclopedia as new varieties were discovered, I inserted fractional numbers in the appropriate places for them. I knew that to find new varieties of this sort, it would require someone with access to a lot of tubes of different years to sort through, and it would take someone with a capacity for spotting very small differences from piece to piece. It can be a very time consuming and tedious thing to search through batches of tubes like that.
What does NOT constitute a new variety is an aberration caused by something getting on the die during minting. Having opened several 'bricks' of 1000 ounces shipped directly from the mint, I had occasion to see what happens. Sometimes the speck of whatever it was that got on the die would be stuck to the first piece struck after it got in there. The speck would leave residue on the die and it would be transferred to subsequent pieces as they were struck, sometimes making a mark on as many as five or ten pieces before it finally wore off, getting fainter with each piece struck. One needs to be aware of these aberrations and what causes them.
There are a number of folk who have contributed to discovering new varieties over the years, and I have given them credit (in the description of the piece they discovered) when they pointed out a suspected new variety to me and I was able to confirm it. There were several instances where Bernard vonNothaus himself denied that a variety existed when I told him about it. He did not think such a thing could have happened (inappropriate obverse and reverse dies being mated in a run). More than once it took sending him an example of the newly discovered piece so that he could confirm it for himself to get him to admit it had happened.
If you think you have discovered a new variety, send me photos and a description of what you think is different and I will try to confirm it. If it is confirmed, I will add it to the encyclopedia and credit you with its discovery.