Definition:  A wrong stock error emerges when coin metal strip intended for one denomination is sent through a blanking press punching out blanks for another denomination.  The resulting blanks end up with the design and denomination appropriate for the size of the blank.  An example of such an error would be a half dollar struck on quarter dollar stock.

The vast majority of wrong stock errors do not involve a change in the coin’s composition.  A quarter dollar struck on dime stock will still have a normal copper-nickel clad composition.

Wrong stock errors involving strip of the wrong composition are very rare.  Why this is so is rather obvious.  It’s easy to switch coils of metal that are identical in every respect except thickness.  But when the stock has a different color on its surface or edge, that difference is hard to miss.

The best-known off-metal, wrong stock error is a 1987-P nickel struck on copper-nickel clad quarter stock.  The
illustrated coin and the small number of other known specimens weigh around 4.2 grams, which is exactly what you’d predict given the thinner cross-sectional profile of the quarter strip.  Photos courtesy of Jon Sullivan

Here a nickel design was struck off-center on a clad planchet that has a straight clip at one pole.  Although no measurements have been taken of this specimen, its diameter seems to match that of a nickel and its thickness seems compatible with a quarter planchet.  It’s almost certainly struck on quarter stock.  In fact, this specimen is probably derived from the leading edge of the same strip that spawned the 1987-P nickels.

This 1971-D quarter was struck off-center on a quarter-sized silver-clad planchet with a large straight clip.  According to NGC, it weighs 2.4 grams.  The composition is said by NGC to be 80% silver and 20% copper.  It is assumed that this is just a measurement of the surface composition.  This ratio of metal is found only in the clad layers of half dollars minted between 1965 and 1970.  However it also appears in the clad layers of half dollar-size Panama 1/2 balboas produced by the San Francisco Mint between 1967 and 1975.

The planchet is indeed silver-clad, with clad layers composed of 80% silver and 20% copper.  A darker core is visible which is presumably composed of 79% copper and 21% silver.  As in typical silver-clad half dollars, the exposed core appears gray.

The planchet upon which this coin is struck is no thicker than a quarter; therefore this is not simply obsolete half dollar stock.  It would appear that a coil of silver-clad strip intended for 1970-D half dollars was mistakenly rolled to quarter thickness, labeled quarter stock, and then stored away with normal copper-nickel clad quarter strip.  Then, in 1971, the off-metal strip was retrieved from storage, sent through a blanking press punching out quarter blanks, and the rest is history.  The presence of the straight clip indicates that this planchet is probably derived from the leading end of the strip.  Since this is a unique specimen, I suspect someone quickly realized the mistake and stopped the blanking press before the strip could progress very far.

While it’s possible that contemporarily 1/2 balboa strip was reduced to quarter dollar thickness, I view this as less likely since the 1/2 balboa coins were struck in San Francisco and because this coin was struck only one year after cessation of silver-clad half dollar production.  In 1970, only the Denver Mint was producing half dollars, and these were restricted to mint sets.