Although these are described as regular die trial pieces in the standard references, the off-metal 'trials' of this year were deliberately made for sale to collectors.
Examples exist in the following combinations:
Copper J772/P857 with only 3 or 4 known.
Aluminum J773/P858 with about a half dozen known. These were offered in full cased sets as in 1868.
Nickel J774/P859 which is probably unique. Pollock has 2 listings which I believe represent the same example.
Photo courtesy of American Numismatic Rarities.