The regular dies trial piece. Examples were struck as follows:
Copper/Silver J428A/P500 struck on the same rolled planchet stock used on 2 cent pattern J407/P477 said to be from native Michigan ore. The unique example shows copper on the obverse and silver with a patch of copper in the upper right quadrant on the reverse and is ex Auction 83, Auction 86, Queller-Heritage 1/09, Rau-Heritage 4/18. According to the 4/18 sale, the coin is struck from "regular dies for business strike 1865 Seated Liberty quarters, Briggs 1-A. The obverse has a Large 1 punched over a Small 1, and the reverse has a crack through the tops of TATE".
Copper J428/P501 which, incredibly, has not been confirmed to exist and is the only silver denomination piece of this year not known to exist as a copper die trial.
Photo courtesy of Heritage.