Judd 521 is an enigmatic "pattern" for an 1866 Five-Cent coinage. We put the word "pattern" in quotation marks because there is strong evidence that these were made outside the mint. In this case, one of the biggest issues is the wide variance from the normal standards of weight and diameter. In 1866, the Mint was busy testing out ways to incorporate Nickel into the regular coins for circulation. A variety of different patterns were produced, but they were all right on or close to the legal standards. The weight of one of the known J-521a's is 72.1 grains, more than four grains off the standard of 76.16 grains (and beyond the 2 grain exception allowed by law. Also, these were struck on oversized planchets (and usually off-center to boot) -- something that did not occur on normal 1866 patterns. The fact that Judd-521a is double headed is also problematic, as the coin has no denomination, and a double-headed coin would never have been considered as serious. Dr. George Fuld argued against these patterns based on the fact that they were not struck as Proofs, which was, by then, a regular practice at the Mint.
Regardless of their source, origin, mode of manufacture, or metallic content, these have now become ingrained into the U.S. Pattern canon and they are extremely popular with collectors. We know of only two Judd-521a's. The example illustrated above came through the Garrett Collection in 1980 where it was listed as a silver Judd 521. Later analysis of the metal determined the actual composition to be nickel (76% copper, 23% nickel). Clearly, any coins of this type should be evaluated for weight, diameter, and metal composition.