The following is a 1798 dollar struck on copper scrap.
The description of this piece is by John Kraljevich and Jim Matthews and is courtesy of Stacks.
"122.2 grains (7.93 grams). 35.6 mm from 9:00 to 3:00 across struck area. 41.7 mm from left angular tip across widest axis. 1.0-1.3 mm thick at struck area, 1.8-2.0 mm thick at unstruck area."
"Two metal detectorists, working with permission on a construction site whose location abutted that of the First Philadelphia Mint, discovered the 1798 dollar struck on copper scrap along with a 1793 half cent in the spring of 2006."
"The dollar trial is struck from a known die marriage in its terminal state, identified as Bolender-30a and BB-116, die state VI. It is struck on a misshapen copper planchet, but shows a significant portion of the surface area of both obverse and reverse dies. Though copper die trials from the 1790s are known on other denominations, the copper 1798 Bust dollar trial is the first to be discovered on that type and denomination. The only other early dollar die trials – both unique – are dated 1794, one with stars and one without. The ground near the First U.S. Mint has been a fertile ground for die trials before. A 1795 half dime in copper, partially defaced, was discovered when Frank Stewart razed the First Mint in 1914; it is now held by the National Park Service. Blank planchets for 1792 silver center cents were found at the same time by Stewart and also reside in the NPS collection."
"The reason for producing an impression in copper from dies that had already produced a substantial number of coins is unknown – it could have been to test the badly broken dies or to show their advanced die state to a member of the Mint staff, or there could have been no reason at all."
"While no Judd number has yet been assigned to this new discovery, the editor of the Judd work on patterns, Q. David Bowers, has reserved the number Judd-25A for this piece in the 10th edition. This falls between Judd-25, a $10 piece of 1797 struck in copper, and Judd-26, a copper trial of a 1799 $10."
"This piece was the subject of a recent column by ANS Curator Robert W. Hoge entitled "A Unique Early U.S. Mint Die-trial Discovery," whose lead line referred to this object as "one of the most exciting items to have
been brought to my attention." He describes the piece further as "recovered from soil in a rubble area, in a vacant lot where modern demolition had occurred." According to Hoge, this piece "will be featured in a future
volume of the American Journal of Numismatics."
It is entirely possible that this is simply a mint error struck on copper scrap that fell between the dies.
Photo courtesy of the American Numismatic Society and Robert Hoge.