The ring gold half dollar of 1852.
This design is actually struck using the reverse die of a half dime with the other side blank with the exception of the denticles.
There are at least 5 known as follows per Akers Gold Pattern book:
2) Chase Manhattan Bank, Smithsonian
3) Boyd, Judd, Wilkison, Paramount, A-Mark, Leidman, 81 ANA, Superior 1/2008 - PCGS65
4) Woodin, Lohr, Cox, Wilkison, Paramount, A-Mark, Auction 79, Auction 82, Simpson collection - PCGS66
5) Sloss, 79 ANA
Earlier sources for #2, #3 or #5 include DeWitt Smith, V. Brand (journal id #46972), Armin Brand (sold 9/1/1940) and ex H.P. Smith (S.H. & H. Chapman 5/1906 lot 1343), Granberg.
2 unstruck blank planchets exist for this, one ex Wilkison and the other ex Sloss, 79 ANA.
A companion gold dollar struck using dime dies also was made as J136/P163.
Photo courtesy of Rick Kay from images taken at the Smithsonian.