This is the reverse die trial of the 1915 Panama Pacific gold dollar struck on a bronzed white metal planchet.
The trial is unique and is listed as ex Keck, Lathrop, 52 ANA, Paxman-Bowers and Ruddy 11/74, Swiatek-Heritage 1/15 FUN - NGC64BN.
There is also a corresponding obverse die trial known, JA1915-1/P3536. To view this splasher, click here.
Photo courtesy of Heritage.