The San Francisco Mint produced two different varieties of the 1981 S Proof Lincoln Memorial Cent (Penny) where one had a filled in S and the other had a Clear S. Depicted in the example image of the coin above is the Type 1 filled S. This can be best identified by the S being filled in and not as clear.
The San Francisco Mint produced two different varieties of the 1981 S Proof Lincoln Memorial Cent (Penny) where one had a filled in S and the other had a Clear S. Depicted in the example image of the coin above is the Type 2 Clear S. This can be best identified by the S being clearer and not as filled in. It is also "flatter" in a vertical sense.