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 J1035/P1171 with only 4 or 5 believed to exist of which at least one has been gilted.
Aluminum J1036/P1172 with only 2 or 3 believed to exist.
Nickel J1037/P1173 which is believed to be unique.
Photo courtesy of American Numismatic Rarities.