The date looks genuine, untampered. I just cut a knick along the edge..and its all silver, no copper. I remember reading somewhere that some 1964 coins were stamped 1966.. I forgot the reason why. It rings like my old quarters that I saved.
Well if it DOES turn out to be a genuine 1966 90% silver quarter you just cut about a $2000 nick in it. Now how about a very inexpensive non destructive test, such as what does it weigh? In grams, two decimal places please.
I don't think I'd ever do that to a coin I thought was a 1 in 108,984,932 coin. I think I'd invest in a digital scale.