This discovery mint error is unique and it is still a mystery why it was struck. After 40 years it is the only known striking of a Kennedy Half Dollar with a composition of 25% Silver and 75% Copper.
As everyone knows, the composition of U.S. silver coins changed from 1964 to 1965. The 1964 Kennedy Half Dollar was struck on a 90% Silver and 10% Copper planchet with a weight of 12.5 grams. In 1965, it was changed to a composition of 40% Silver and 60% Copper with a weight of 11.5 grams.
This unique off-metal is on a full-size planchet with a weight of 7.62 grams and has a composition of 25% Silver and 75% Copper.
It does not match any known world coin struck at any of the U.S. Mints during the time period of the transition from 90% Silver coinage to 40% Silver coinage. Nor does it match the weight or composition of the inner core of the planchet strip used to strike the silver coinage of 1964 or the silver clad coinage from 1965 through 1970.
It is unknown why this piece was struck and why a planchet was used with this weight and silver composition.