This unique off-metal Kennedy Half was just authenticated and certified by NGC as Struck on a Copper-Zinc Planchet, 83% Cu, 17% Zn (2.4 grams).
This is a fascinating and enigmatic mint error. No foreign coins struck by the U.S. Mint match anything even close to this composition and weight. The Mint Error News website has a 69 page report of coins struck by the U.S. Mint for foreign countries. It is the most comprehensive report available anywhere. After an exhaustive search, no known planchets match the composition of this mint error. Click here for a pdf of the 69 page report.
NGC published an article discussing the U.S. Mint experiments for alternatives to the existing composition of 95% copper and 5% zinc. Bronze-clad steel cents that were tested in the late 70's were not satisfactory, so further tests were conducted during 1980-1981. The price of copper forced the U.S. Mint to replace the cent with an alternative composition. The final composition was 97.5% zinc and 2.5% copper. These new zinc cents were struck on January 7, 1982 at the West Point Mint. Soon thereafter, Philadelphia, and then Denver, struck these as well. Click here for the full NGC article.
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The weight of this Kennedy Half off-metal is within tolerance for the new 1982 Cents weighing 2.5g but the composition doesn't match. Furthermore, since it does not match any composition or weight of a foreign planchet that the United States struck for a foreign country, it likely was struck on an experimental copper-zinc planchet during 1980-1982 when the U.S. conducted tests for the new cent planchet.
The U.S. Mint used the Martha Washington dies during this period to test the new copper-zinc planchets for 1982. Below is an example of one of the Martha Washington dies and cent tokens from this test. Click here for more info.
Martha Washington Test Strikes
 
This Martha Washington Test Piece was struck to test the new copper-zinc planchets for the U.S. Mint in 1982. This coin is the plate coin for Judd #2180 on page 294 of the eighth edition of United States Pattern Coins, Experimental and Trial Pieces. Below is the only known Martha Washington Reverse Die in private hands for ANY denomination. Click here for more info...
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The U.S. Mint had previously conducted metallurgical testing done by the Philadelphia Mint for an alternative to the pre-1943 bronze cent. Below are examples of some of the experimental planchets. Click here for more info.
This unique Kennedy Half off-metal, likely struck on an experimental cent planchet, not only offers some insight into the U.S. Mint testing process but is also a spectacular mint error. To date, there are no Kennedy Halves known, in mint state or proof, that were struck on the new 2.5 gram Lincoln cent planchet. This discovery belongs in a collection of world-class Kennedy Half mint errors or a collection of unique numismatic rarities.
Featured on the cover of
Mint Error News Magazine Issue 71:
Off-Metal Errors are featured in the NLG award winning book,
World's Greatest Mint Errors.
