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 J1348/P1492 with only 3 confirmed.
Aluminum J1349/P1493 with fewer than 3 confirmed.
Photo courtesy of PCGS and is believed to be the former Lohr-Polis example sold in the B/M 6/91 sale.