Although described as a regular dies trial piece of the 1864 half dollar struck in aluminum, it is likely that it was deliberately struck for sale to collectors.
An example was in the Idler collection, he being one of the Mint's fences for supplying rarities and profit for 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 J538/P602.
The following 2 examples are confirmed:
1) Superior 8/91 as NGC64, Bowers and Merena 3/95, Heritage 6/18 - PCGS64, illustrated above courtesy of Heritage. Click on the thumbnail image to see an enlargement.
2) Simson collection - PCGS65, illustrated below courtesy of PCGS.
Additional listings for these 2 pieces include: Idler-Haseltine 4/1870; Col Cohen (Cogan 1875)-Parmelee (1890); Newcomer-Morgenthau 4/32, Elder 3/38; Lohr-Cox-Stacks 4/62. It is not possible to determine which pedigrees go with which coin.