OK So this coin is definitely not uncirculated, meaning its seen circulation, its just not seen as much circulation as other quarters from the same year 1966. They didn't make proofs in 1966 they made SMS (Special Mint Sets). Special mint sets are not quite proofs, but struck cleaner and taken better care of than general circulation coins. That being said I would need to see it in person to tell you much more. I might stop by a coin store on my way to work or when you are out and about, and just hop in and ask them to look at it. Coin dealers are usually pretty nice and easy going, and would probably love to discus the coin further with you. My guess is that...it was either SMS or was part of someone's collection, that at some point after a few decades decided to spend the coin at face value and it went into circulation. Hope that helps! Thanks for posting, if you find something else let us know :)