The Kennedy Half Dollars that are 40% silver were struck from 1965 through 1970. The outer two layers are silver, the center core is copper. It is rare when these are struck missing an outer silver layer. This can occur in two ways. Either by an outer silver layer splitting off prior to being struck or the blank planchet was rolled missing the outer silver layer.
These are not to be confused with the 'clad layer missing' mint errors, which were struck from 1971 to the present day. The clad missing layer mint errors are scarce but not rare. The silver missing layer mint errors are rare due to a short type series (1965-1970) and due to the fact that silver planchets seldom split prior to being struck.