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 J1379/P1524 with only 2 or 3 known.
Aluminum J1380/P1525 with only 2 or 3 known, one of which shows corrosion on both sides.
Photo courtesy of PCGS.