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 with the copper ones sold as part of complete sets.
Examples were struck in copper J1482/P1635 and aluminum J1483/P1636.
Only 1 or 2 are known of each. Those listed in Pollock have been gilted due to the extreme rarity of its gold counterpart. I believe all of Pollock's listings for the aluminum piece represent the same piece. It is unknown if either of these exist in an ungilted state. A gilt set is in the Simpson collection.
We have also seen 2 different counterfeits.
Photo courtesy of GreatCollections Coin Auctions.