This is the first proof coin that was accidentally struck by the United States Mint without containing the S mint mark.
Only a few dozen examples exist in all grades combined, making 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 aren't any 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 has surfaced in a public auction a total of only 19 times in the past 12 years. This means that anyone who is interested in purchasing this coin, will have to compete for the one or two 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. For instance, in 1997 a PCGS PR68 example sold for approximately $6,000. In 2005, the coin in the same grade of PCGS PR68 sold for $32,200 or an increase of at least five fold within an eight year period.
The coins were struck very well and they exist mostly in the grades of PR67 and PR68. Coins displaying a cameo designation are extremely scarce, PCGS has certified a total of only five examples in all grades with the cameo designation. 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 the best example 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 through out the years. This is definitely a special coin that deserves to be in any great collection.