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 J1164/P1306 with fewer than a half dozen confirmed.
Aluminum J1165/P1307 with fewer than 3 confirmed.
Nickel J1166/P1308 with only the Farouk, Pine Tree 3/74, Paramount 4/77 coin believed to exist.