you are lucky NGC let it be a registry coin. Afterall it's not legal tender and sometimes NGC won't let legal tender coins in such as error's. Take for example
the 1990-s no S proof cent. It's a 10k coin but it gets a single point on the registry scale. Being an obvious error coin the 90 no S by ngc definition shouldn't be allowed at all. BUT it is a notable error coin and has recognition. NGC has allowed certain notable error coins be included in the registry some like the 90 no S get a minimal point others get full point status for the grade. I think NGC does this in an effort to make sure the coins in registry sets are indeed collectible. For sure your coin is sweet and as a collector of three cent nickel proofs i am indeed admiring. Registry justice and rarity are not always synonymous. I will say that i think NGC indeed tries to make it a level playing field. In any kind of competition that's about the best one can hope for. Hope it helped. It's just my opinion based on certain coins i have seen NGC let in the registry and how it was handled. Once again i think NGC was judicious and even expeditious in explanations and decisions. If it helps you can ask, as i know for a fact they will answer.