Judd 666 is a pattern for a Cent made of Copper-Nickel. It is basically a miniature version of the 1869 Three-Cent Nickel, except with a Roman numeral I on the reverse. Three minor varieties exist, none of which are enumerated by Judd, but all of which are given separate listings by Andrew Pollock. Unfortunately, we know of few collectors who collect patterns by minor varieties.
Judd 666 is a common coin in the global world of pattern coins. PCGS alone has certified over three dozen examples. According to pattern researcher, Saul Teichman, Judd 666's were sold in sets along with 1869 Three-Cent and Five-Cent patterns, which may explain why so many exist today.