What this coin looks like (obverse, reverse, mint mark location, special features, etc.): One major variety of the 1879 S Morgan Silver Dollar features the reverse design of 1878. The San Francisco Mint produced two major varieties including a coin using the later reverse of 1879.
The reverse of 1878 is actually the earlier design which can be identified by noticing how the eagle had an unnaturally flat or concave breast. The easiest way to identify this variety is to look at the arrow feathers on the arrows that the eagle is holding in it's claw. The top feather is straight and parallel along with the rest of the arrows.
The later reverse of 1879 variety was changed to give the eagle a rounder and more convex breast appearance that looked more natural. Again the easiest way to identify the reverse of 1879 is to also look at the arrow feathers. Unlike the reverse of 1878 - the reverse of 1879 features a prominently slanted top feather on the arrow. The difference between the Reverse of 1878 vs Reverse of 1879 Morgan Silver Dollar is depicted below in the example comparison image with the features depicted by the arrows:
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