Although described as regular dies trial pieces, this was more likely deliberately struck for sale to collectors as part of complete copper sets.
T. Harrison Garrett had one and a double set was apparently stolen from the Iowa State Historical Museum.
Examples are known as follows:
Copper J558/P619 with about half a dozen known, including one non-proof example.
Oroide J559/P620 with only 2 or 3 known.