I got this 1879-S Reverse of '78 on ebay. According to the Top 100 Vam book, there are 15 known 1879-S varieties with the reverse of '78. Charlie
It has to do with the arrows, if you look at the ends of them, and compare that to a morgan from a different year, you should be able to tell the difference. Also, if it was uncirculated, you could tell by the eagle's breast, which would be flat.
The reverse of '79 has a slanted top arrow feather, while the reverse of '78 is straight. The one on the left is the reverse of '78 and the one on the right is the reverse of '79. Charlie