I can't help it but myself like some others question as to how this Quarter was struck on a Silver Nickel Planchet! The only time that a Nickel (not a Half Dime) were ever stuck on (partial) Silver Planchets was from 1942 to 1945 and these were only 35% Silver in content. The coin in question appears to be on a 90% Silver Planchet of which none should exist or have existed. Did some Silver Dime Planchet Roll stock get inadvertently run through the Planchet Cutters (Punches) when the Mint was punching out Nickel Planchets? If so, then this could possibly explain the existence of this coin! Did PCGS screw up when they noted the Attribution of the Error on the holder's (slab's) Label? This surely could be the case and the coin was actually struck on a Silver Dime Planchet intended for 1964-D Dime production! However, in my mind, if the Attribution on the holder's (slab's) Label is correct, then I feel that this could have resulted from a case of Mint Employee intentional help (manipulation). I am not trying to take anything away from the O.P.'s coin but such an Error due to Mint Employee manipulation is not a true Mint Error!
samir4sap... I am sorry to steer away from the significance and awesomeness of your' Error coin! However, due to it's existence and the Attribution on the Label, makes many of us and especially me question such an Error as I have found, purchased and sold thousands of Error and Variety coins over many years. I found my' first and only 1955-P "Double Die" Lincoln Cent DDO-001 in 1973, two 1972-P "Double Die Variety" Lincoln Cents (not the DDO-001's) in 1976 and have been searching, finding, purchasing, attributing and selling and/or reselling Error and Variety coins ever since!
Frank