The 1968-S Proof Dime is the first date for the Roosevelt Dime Proofs where a coin was missing the "S" mint mark. There are other coins that are lacking a mint mark but these are usually a result of over polishing or grease filled dies. For the 1968 No "S" Proof Dime, more than likely the missing mint mark was never punched into the die when the coins were originally struck.
This mistake must have been caught very early in the production process by mint employees, as only a few examples managed to escape. This was also the first date for any series of U.S. coins where the "S" mint mark was omitted.
Since its discovery, the 1968 No "S" proof dime has not only proven to be scarce but has performed financially well in the coin market.
If one would have acquired an example back in 1999 it would have proven to be a wise decision. For example, in 1999 a PCGS PR 68 example realized $6,038 at a major auction. Since then, the 1968-S "No S" Proof Dime has appreciated efficiently in price. The most recent sale in 2011 saw a PR 67 example (which is a full point grade lower) sell at a Heritage Auction for $21,850. One prior sale was in 2006 for a PR 68 Cameo example which fetched $48,875 at auction. However, it might be a while until another example makes its next appearance since no examples have surfaced at auction since February 2011.
Walter Breen's "Encyclopedia of US Coins" reported six known at the time of publishing. However, over time several more examples have surfaced and it is now believed that there are at least one to two dozen coins in existence. Currently the PCGS Population Report shows 21 examples in all grades: 14 total coins have been graded without the Cameo designation. For the Cameo designation, there have been a total of seven coins graded in all grades combined.
Whatever examples exist today, we know for certain it is a very small quantity. We may never know for sure the exact amounts that do exist but owning an example with such intriguing history behind it can be a great story of its own.