Although described as regular dies trial pieces in the literature, it is more likely that these were deliberately struck for sale to collectors.
Examples are known as follows:
Copper J554. This is unlisted in Pollock. If any do exist, they are probably masquerading as a regular issue proof. Examples were included in the double set that was apparently stolen from the Iowa State Historical Museum. As the whole set is missing, it can not be checked to see if they were really copper or regular bronze strikings. According to Scott Schechter of NGC, the cent in the recently discovered set in Europe was tested and found to be 99.7% copper.
Copper-nickel J555/P616 with only the Brenner-1914 ANS exhibit piece mentioned.
Nickel J556/P617 with only 2 or 3 believed to exist.
Oroide J557/P618 with only the J.E. Drew piece mentioned.
Photo is of the former J.E. Drew piece.