Although these are described as regular die trial pieces in the standard references, it is more likely that these trials were deliberately struck for sale to collectors as part of complete off-metal sets.
Examples were struck as follows:
Copper J1238/P1380 with fewer than a half dozen known including the illustrated example below. Photo courtesy of Superior.
These were also struck in aluminum J1239/P1381 with fewer than a half dozen known.
Photo courtesy of Heritage.