This famous piece is a 1795 draped bust silver dollar from Bolender 14 dies apparently struck on a half cent planchet. This piece has been described as a setup trial piece and also a mint error. Adams and Woodin listed it as AW18, Judd lists it as a mint error.
The piece is unique and is ex Woodward 4/1863, Parmelee, Woodside, Woodin-1914 ANS, Newcomer, Boyd, Kosoff 7/51 Numismatic Gallery Monthly, Kosoff 5/55, Judd, Hydeman, Stacks 5/75.
Stacks called it a "setup trial piece to insure the striking distance between the obverse and reverse dies" in their 5/75 sale but a half cent is not as thick as a dollar die so this explanation is dubious.
We are listing it here as a mint error because we see no reason to create a setup trial piece on a planchet that is smaller and not as thick. The mint was able to create full size dollar planchets in copper for the 2 1794 dollars J18/P27 and J19/P28 and all other die trials of this year are full-sized as well.
Photo is a scan from the Stacks 5/75 sale.