Some of the state quarter marketing gimmicks were to plate the years and sell them by subscription, I have seen silver, gold, and platinum plated coins, the weight should be the regular weight for the clad coinage.
Probably plated. Philly wouldn't have had any silver quarter planchets on hand to be struck accidently.
Silver or platinum plated. Millions of them were so defaced. Does the other two sides have a peculiar look? A different luster and color?
likely broken out of a set of platinum plated statehood quarters... one of my biggest pet peeves of all time (platinum plated clad coins)
True but since the quarters have a weight tolerance range, you don't know what the coin weighed before plating, also the weight of the plating is less than the mint tolerance. So for all practical purposes the plated coin weighs what an unplated one does. See the coin listed above that weighs between 5.7 and 5.8 grams. Spec with tolerance allows up to 5.9 for an unplated one.