Only two examples of the 1975-S No S Dime are known. The first coin was discovered in a 1975-S Proof Set in 1977. The second example was found seven years later. The discovery coin was sold in 1979 to Proof coin specialist, Fred Vollmer, who sold it the next years later to a collector. Neither coin had ever appeared at auction until the discovery coin was included in the 2011 ANA sale offerings by Stack's-Bowers.
"No-S" Proof coins were the product of the way in which Proof dies were handled. Beginning in 1968, when the San Francisco Mint began striking Proof coins, all dies for Proof coins were prepared at the Philadelphia Mint. This was done by adding a mintmark to a normal P-Mint die. However, some dies escaped this process, thus we see "No-S" dimes in 1968, 1970, 1975, and 1983, "No-S" Nickels in 1971, and "No-S" Lincoln Cents in 1990. Any estimates of the number of "No-S" coins is strictly a guess, but all except for the 1975 No-S Dime have certified populations of between 24 and 242.
The extreme rarity of the 1975 "No-S" Proof Dime can be explained in two ways (both plausible conjectures):
1. The error was discovered at the Mint in time to stop the press, but not before a very small quantity of sets had already gone out the door.
2. The coins were deliberately made. This explanation is not as far-fetched as it may seem, because other remarkable Proof errors emerged from the San Francisco Mint during the early 1970's (for example, a Proof 1970-S Washington Quarter struck over a 1900 Barber Quarter -- from Philadelphia)!
Sources and/or recommended reading:
Gibbs, William T. "Rare Proof Dime Error To Make Auction Debut", COIN WORLD, June 6, 2011.
There are only two 1975 No S Proof Dimes known. The coins were first discovered in July 1977 when the first example was sent to Coin World in the original 1975 Proof Set that it was purchased in.The following year, Anacs certified this coin. A few months later Anacs certiffied the second known example.
The 1975 No S Proof Dimes were struck with regular Proof Dies for that year, with the exception that the obverse die lacked the S mint mark. The only example graded is a PCGS PR68 that was submitted to PCGS in June 2011. This is the same coin that was submitted to Coin World in 1977. The coin will be auctioned off by Stacks Bowers Galleries' at the ANA on August 16 - 20, 2011.
This is the first time a 1975 No S Dime has surfaced in a public auction.
1975 Proof Set: The 1975 Proof sets had a mintage of 2,845,450. Each set had a price of $7 each. Each set contained the following coins 1975 Lincoln cent, 1975 jefferson Nickel, 1975 Roosevelt Dime, 1976 Bicentennial Washington Quarter, 1976 Bicentennial Half Dollar and finally the 1976 Bicentennial Half Dollar.