Although these are described as regular die trial pieces in the standard references, it is more likely they were deliberately struck for sale to collectors as part of complete off-metal sets.
Examples were struck as follows:
Copper J781/P866 with about a half dozen known.
Aluminum J782/P867 with about a half dozen known.
Nickel J783/P868 This is unique with only the Farouk, Stacks-Bowers 6/11 piece believed to exist.
Photo courtesy of Heritage.