This is a metallurgical trial piece struck in platinum from the regular half dollars dies of variety Overton 107 per Pollock.
Only 3 are believed known as follows:
1) Smithsonian with test cut on edge at 11:00 - illustrated above. Click on the thumbnail image for an enlargement.
Photo courtesy of the National Numismatic Collection of the Smithsonian Institution.
2) R.Coulton Davis, Woodside, Virgil Brand (journal #13866) to Armin Brand to B. Max Mehl (May 5, 1933 invoice at $900), Col Green, Judd (per 8/6/43 B.G. Johnson invoice), Pine Tree 9/74, Jelinski, Auction 88, 91 ANA, Bowers and Merena 11/02, Heritage 8/11. This piece has 33 "P"s punched on the obverse and the word "platina" engraved on the reverse. Illustrated below.
Photo courtesy of Bowers and Merena.
3) Nagy, Nick Parker per Pine Tree 9/74 catalog. This is also apparently the source used by Taxay in his 1976 Comprehensive Catalog.
According to the Bowers sale catalog, these were unquestionably struck in 1814. This is based upon obverse and reverse die state analysis combined with an analysis of the edge device.