It could be what we call a "filled mint mark." Consider: features that are "positives" on the struck coin are "negatives" - voids - on the die, and vice versa. Stands to reason. Now, the inside of a mint mark amounts to a really thin post of metal on the die, and under the repeated violent events of thousands of strikes, they can break off. The mint mark then becomes filled with planchet metal, a solid chunk instead of a letter with a hollow center. It can happen to any die at any time, and is pretty common. However, when one looks at the displaced of metal to the north of the mint mark, what kind of looks like doubling above it, one can also believe that an impact in the right place just "smeared" metal, covering the opening. 50/50, and in either event not a value-added proposition.
Thanks for the educational response I am learning. I'm not a collector per se but a treasure hunter in my grandfather's numismatic collection. However I don't know that there is much of a difference. Lol
FYI - The Bicentennial Quarter is one of the most collected of all quarters, not because it has any real numismatic value, but just because it is different. Your grandfather, like millions of others, probably saved it for that very reason. Chris
