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 J1233/P1375 with fewer then 4 confirmed to exist.
Aluminum J1234/P1376 with fewer then 4 confirmed to exist.
Photo courtesy of Bowers and Merena.