Although described as a regular dies trial piece of the 1864 half dollar struck in aluminum, it is likely that it was deliberately struck.
It was originally in the Idler collection, he being one of the mint's fences for supplying rarities and profit to mint personnel.
It would be interesting to find out if this reverse die matched the one used on the famous 1866 no motto half dollar a J538/P602. This could be a later restrike although not much later as it has the following pedigree. Idler-Haseltine 4/1870, Col Cohen (Cogan 1875), Parmelee (1890), Newcomer-Morgenthau 4/32, Elder 3/38, Lohr, Cox-Stacks 4/62, Superior 8/91, Bowers and Merena 3/95 - PCGS65.
Photo courtesy of PCGS.