Pattern coins as the name suggest are typically minted during a major revamp of the coin design. The reason they are called Pattern coins is that they are minted so that relevant decision makers get to see the actual coin before the design is adopted and then the coins are put into general circulation. If the design is rejected, then these coins are not put into circulation and hence are called pattern coins. In the modern times there are a bunch of coins stamped “EXPT” supposed to mean Experimental coins.
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