In 1981 two varieties of “S” mint-marks can be found for all proof coin denominations.  The minor details will vary from coin to coin so any coin you look at may not look exactly like the picture below, however the following characteristics are what you would use to determine which variety it is.

The first one is commonly called type 1, and is the same mint-mark that was used in the later part of 1979 and all of 1980.  This mint-mark has smaller and more rounded serifs (the bulbous or ball like formations at the beginning and end of the “S”) [1].  If you look at the mint-mark you will notice it has a dome shape like a bead of water [2].  The center loops of the “S” are also more of an oval shape [3].

Type 2 proof coins were minted starting later in the year.  The mint-mark has larger serifs [4], and these serif bulbs tend to look flattened.  You will also notice that the mint-mark has more of a plateau or more flat surface than the type 1 [5].  The center loops have more of a circular shape [6].

Note:  Some type 1 coins have very clear mint-marks while some type 2 coins do not, so how clear or well struck the mint-mark is does not indicate the type.

Pictures coming soon

 
1981 S Type 1 Nickel         1981 S Type 2 Nickel
1. Smaller serif                       4. Larger serif
2. Dome surface                     5. Flat surface
3. Oval center loops               6. Circular center loops