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 in complete sets.
Examples were also struck as follows:
Copper J766/P851 with about a half dozen known.
Aluminum J767/P852 with about a half dozen known.
Nickel J768/P853 with only the former Farouk, 76 ANA example believed to exist.
Photo courtesy of Stacks.