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How would 1955 planchets get minted in 1954?
Many different ways it could happen. Almost all websites define a transitional planchet error as a coin that is minted with a design for an earlier or later year planchet so it's possible. My guess would be during the switch of planchets and dies, a 1955 planchet made its way through while the 54 dies were were still in. How most transitional planchet errors occur.