Although described as regular dies trial pieces, these were actually deliberately struck for sale to collectors as part of complete off-metal sets.
These were struck from the closed 3 obverse die.
In aluminum, as illustrated, only 2 or 3 are believed to exist.
A single example struck in copper on a thick planchet J1258/P1399 is known. It is ex Farouk, Bolender 3/55 sale lot 1477. The Bolender pedigree was inadvertently placed on the aluminum example in Pollock.
Photo is of the former J.E. Drew example.