A small number of proof coins struck at the San Francisco Mint for inclusion within the 1971 Proof Set were mistakenly produced without the “S” mint mark. Statements later provided by Mint officials established the number of 1971 “No S” Proof Jefferson Nickels at 1,655. This represented only a minuscule portion of the total 3,220,733 proof sets sold by the Mint.
Collectors might want to double check their 1971 Proof Sets to see if the rare nickel is included, although chances are slim.
Similar situations of the mint mark missing from modern proof coins had occurred with the 1968, 1970, 1975, and 1983 “No S” Roosevelt Dimes.
taken from: http://jeffersonnickel.org/