The 1851 restrike dollar struck in copper. These were deliberately struck for sale to collectors. Early 2001 articles in Coin World by Dave Bowers discuss practices by the mint in those years.
Silver restrikes of this date were known as early as lot 134 of Cogan's May 1859 Simon Gratz sale but the earliest occurrence for one of these copper pieces appears to be lot 1995 of Cogan's October 1865 F.S. Edwards sale. According to the Heritage cataloger, the illustrated piece, ex ANR 9/03, Goldberg 6/04, Goldberg 9/10, Heritage 5/23, was struck from the Osburn-Cushing P4 reverse which was first used on 1865 proof dollar does place that date as a likely time for a second run of silver restrikes during which a few of these copper pieces were also included. Additional research is needed here to see if other reverse dies were used. It is known that the Mint had at least 2 "No Motto" reverse dies on hand in the 1870s.
In copper, only 7 or 8 examples are confirmed. To see images of all of these, click here.
Virgil Brand owned 4 (journal #18146 Low 3/1898, #40712 Elder 10/14/1907, #44166 Elder 6/15/1908 Gschwend and #82007 Elder 2/23/1917) and Waldo Newcomer owned one.
There is also a single example known struck in nickel J133/P160 formerly from the Newcomer and Farouk collections. Since nickel wasn't really used on dollar patterns until 1869, this may have been struck along with a third batch of silver restrikes circa 1869-1871 or may be a plated copper piece.
Additional information can be found in "Silver Dollars and Trade Dollars of the United States" by Dave Bowers and Mark Borckardt.
Photo courtesy of American Numismatic Rarities.