Ron Guth: 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 year to a collector. Neither coin had ever appeared at auction until the discovery coin was included in the 2011 ANA sale offering by Stack's-Bowers, where it sold for $349,600.
"No-S" Proof coins were the product of the way 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 1970s (for example, a Proof 1970-S Washington Quarter struck over a 1900 Barber Quarter – from Philadelphia).