The 1971 No S Proof Jefferson Nickel was accidentally struck with a proof die that was missing the S mint mark. This is the only coin in the entire Jefferson Nickel series that has been struck without containing the intended mint mark.
The coins were found in 1971 Proof Sets and there are possibly several hundred examples known. In Proof without cameo or deep cameo surfaces, there are possibly several hundred in existence. With cameo surfaces there are probably also a couple hundred examples known. Finally, with Deep Cameo surfaces they are very difficult to come by and possibly no more than 50 examples exist displaying the Deep Cameo surfaces.