What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
The San Francisco Mint produced two different varieties of the 1979 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. Below is a close-up image comparing the Type 1 Filled S vs the Type 2 Clear S. Click the image to enlarge:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD