Could be a lamination defect in the planchet or post-mint damage that has been worn down with the rest of the coin. Either way this is a classic "filler" that can plug that hole in a date set until a nicer example comes along.
This might be a good coin to give to a youngster just getting interested in coins. The potential benefits for inspiring a young collector are far greater than the $0.50 or so you might get from selling it.