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 J1335/P1479 with only 3 or 4 known.
Aluminum J1336/P1480 with only 3 or 4 known.
Photo courtesy of Bowers and Merena.