Nice find! Now it's called an "impaired proof" coin. An impaired Proof coin is a Proof coin that grades less than PR-60, meaning it has been circulated or shows significant wear despite originally being struck as a Proof. Proof coins are typically struck with extra care, multiple strikes, and on specially prepared dies and planchets to produce sharp details and mirror-like surfaces. However, if a Proof coin has been handled or circulated, it loses its pristine condition and is considered impaired. Not sure about how many people know this.
