Uniface Die Trials are deliberate, one sided impressions using either the obverse or reverse die. These Die Trials can be from finished or unfinished dies. Sometimes hubs are used, rather than the actual finished die. Metals other than the adopted composition are frequently used to strike Uniface Die Trials.
Above is a unique 1857 U.S. Half Dollar Uniface Die Trial in white metal. It is designed by Paquet, who created many of the patterns during that time period for the U.S. Mint. This design shows an eagle with a ribbon in its mouth with 13 stars above, and the denomination "Half Dollar" below. This is a fascinating piece of Numismatic History because it is an unfinished die trial. It is uniface (being struck by only one die on one side) on white metal which was not adopted as an official composition by the U.S. Mint.