Although described in the literature as "regular dies trial pieces", these are actually restrikes deliberately made in the early 1870s and sold as part of complete off-metal sets with the silver coinage of this year containing quarter, half dollar and dollar which had the reverses of 1866 with the motto "In God We Trust" above the eagle.
These half dime dies were made from a hub which had the "D" in "United" broken and are believed to be backdated novodels.
Examples were struck as follows:
Copper J323/P388 with about a dozen known.
Aluminum J324/P389 with at least 3 confirmed.
Photo courtesy of Heritage.