Although these are described as regular die trial pieces in the standard references, it is more likely that the off-metal 'trials' of this year were deliberately struck for sale to collectors.
Examples were struck as follows:
Copper J1170/P1312 with fewer than a half dozen believed to exist.
Aluminum J1171/P1313 with only 3 or 4 believed to exist
Nickel J1172/P1314 which is believed to be unique as a pattern. There is a second example in AU condition ex Heritage 8/2001 ANA, Heritage 4/18 that may be a mint error struck on a shield nickel planchet.
Photo courtesy of American Numismatic Rarities.