These are listed as regular dies trial pieces. Some of these may be mint errors.
Examples are listed as follows:
Copper J1757. This was delisted by Pollock. A circulated example, struck on a thick planchet, was sold in Stacks 12/97 sale and a second weighing 67.9 grains was sold in Bowers and Merena's May 2006 sale. The weight is within the tolerance for these to simply be mint errors struck on misrolled copper planchets of nickel thickness stock.
Nickel P1970A. This was unknown to Judd. Pollock lists an example weighing 46.4 grains struck in standard coin nickel (25% nickel, 75% copper) which is probably a mint error struck on a planchet for a South American or Haitian 5 cent piece.
Copper-Nickel J1758/P1971 with at least 3 known. The illustrated example above is from Heritage 1/97. Remember to click on the thumbnail image to see an enlarged picture. Two others are circulated. These are probably struck on planchets intended for a medal or foreign coin and appear to be the same as on the 1891 Liberty nickel wrong planchet mint error illustrated below.
This piece is 86.9% copper and 13.1% nickel.
Photo courtesy of Rich Schemmer.
Aluminum J1759/P1972 which is described as unique in Taxay. We have not been able to confirm its existence.
Photo courtesy of Heritage.