Off-metal 1964 Jefferson nickels truck on a Lincoln cent planchet, and uniface reverse. The error occurred when a cent planchet was fed into a press striking Jefferson nickels and was struck while it was on top of a Jefferson nickel planchet. The result is a nickel on cent planchet with the cent planchet having expanded to the normal diameter of a nickel (the expansion was caused by the increased pressure exerted by the nickel planchet beneath it), and with a uniface (no design) reverse. Rare, and the only example we can recall seeing exactly like this.
PCGS Uncirculated details, with a cut mark from 9 O'Clock inwards to the center of the coin. It is toned over, so not as obvious.