Transitional errors: Coins with a high coolness level that await discovery

?Kennedy half dollars are back in the news, thanks to the discovery of a rare transitional error for the 1971-D Kennedy half dollar, as reported by Paul Gilkes.
Transitional errors generally occur when a mint is making a change in composition. They have a coolness level that is hard to match, particularly since they can be found in circulation when the coin’s composition is in the process of being changed, or even discovered years later as with this newly reported piece.
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By 1970, however, silver’s value had risen to the level that even a content of 40 percent was not viable. The last half dollars struck for circulation in a silver alloy were the 1969-D coins. In 1970, no half dollars were struck for circulation, though the 40 percent silver composition was used for the 1970-D coin for the Uncirculated Mint set and for the 1970-S half dollar for that year’s Proof set. In 1971, when production of the denomination resumed for circulation, the half dollar was made of the same copper-nickel clad composition as the dime and quarter dollar.
The discovery more than 40 years later of a 1971-D Kennedy half dollar presumably struck on a leftover 1970 silver-copper clad planchet is a reminder that great coins can still be found.
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Transitional errors: Coins with a high coolness level that await discovery