This is the first proof coin that was accidentally struck by the United States Mint without containing the S mint mark.
There are only a few dozen examples known in all grades combined, making it an extremely scarce coin. Because of the small amount of coins in existence, it is very likely that the San Francisco Mint caught these mistakes very early in the production process. There are no Mint records providing any details for these coins, which makes them a complete mystery.
Several years later, the Mint also produced 1975 No S dimes which are even scarcer than the 1968 No S Dimes, (there only two 1975 No S dimes known). Other proof coins that are missing the S mint mark, include the 1970 No S dimes, 1971 No S nickels, the unique 1976 No S Ike and the very popular 1990 No S Lincoln cent.
The 1968 No S Dime does not seem to come up for sale often. The auction prices realized indicate that the coin is usuallt ffered at auction only a few times a year. This means that anyone who is interested in purchasing this coin, will have to compete for the few coins that do appear at auction every year. There is unquestionably a lot of demand for this coin. Perhaps this is why the coin has performed very well throughout the years.
Coins displaying a cameo designation are extremely scarce. Coins displaying a deep cameo designation are not known and if someone does find one, it is highly recommended that it be certified by PCGS as this would probably be one of the best examples in existence for this very popular coin. This coin has it all, scarcity, popularity, mystery, demand and not to mention, it has performed very well throughout the years. This is definitely a special coin that deserves to be in any great collection.
The 1968 No S Proof Dime is a modern rarity and one of the most dramatic errors the Mint has ever made. The "error" in this case is that the Mint left the mintmark off the die when it was prepared in Philadelphia before shipment to San Francisco. This has turned out to be one of the rarest of the "No S" Proof coins, exceeded only by the 1975 No S Dime (of which only two are known). Since its inception in 1986, PCGS has certified only 18 1968 No S Proof Dimes, 12 of which are brilliant Proofs and 6 of which are Cameo Proofss. As of July 2011, the finest examples certified by PCGS include five PR68s and 3 PR68CAMs.