Although described as a regular dies trial piece in the literature, it is more likely that this was deliberately struck for sale to collectors as part of complete off-metal sets.
Examples were struck as follows:
Copper J787. This is unlisted in Pollock. If any do exist, they are probably masquerading as a regular issue proof.
Aluminum J788/P875 with only 2 or 3 believed to exist.
Nickel J789/P876 and is believed to unique.
Photo is a recolored example of a regular proof and is courtesy of Heritage.