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 J1172/P1315 with about a half dozen known.
Aluminum J1173/P1316 with fewer than 3 confirmed.
Nickel J1174/P1317 which is believed to unique.
Photo courtesy of Heritage.