Although these are described as regular die trial pieces in the standard references, it is more likely that the off-metal 'trials' of this year were deliberately struck for sale to collectors as part of complete denominational sets.
Examples were struck as follows:
Copper J873/P970 with about a half dozen known.
Aluminum J874/P971 with only 2 or 3 known.
Nickel J875/P972 which is possibly unique - formerly in the Dunham and Farouk sales.
Photo is of the G.H. Wolfe speicmen and is courtesy of Heritage.