Although described as a regular dies trial piece, these were actually deliberately struck for sale to collectors as part of complete off-metal sets.
Examples were struck as follows:
Copper J1414/P1557 with about a half dozen known.
Aluminum J1415/P1558 with at least the following 2 confirmed to exist:
1) B/R 9/75, NERCA 12/76, Heritage 1/18 FUN - PCGS64
2) Crouch-Superior 6/77, Heritage 4/12 - PCGS64
Photo courtesy of Heritage.