The first reverse had 8 tail feathers. It was then pointed out that eagles only have 7 tail feathers, not eight. So the mint began fixing that oversight.
A bit more info from coinfacts.com:
After using up the 8 Tail Feathers and 7/8 Tail Feathers dies, the Mint finally settled down to the anatomically correct 7 Tail Feathers version. However, two additional problems required a fix. One was the eagle's breast, which appeared unnaturally flat on the earliest 1878 Morgan dollars. The other was the top fletch in the bundle of arrows on the reverse; early versions had a parallel fletch that was out of perspective with the way the arrows were aligned. These two problems were fixed on the reverse dies used in the final part of 1878 - the eagle's breast became rounded and the top fletch was slanted. The dies became known as the "Reverse of 1879" to differentiate them from earlier ones. Thus, the 1878 7 Tail Feathers can be found with either a Reverse of 1878 or a Reverse of 1879!
A bit more info from coinfacts.com:
After using up the 8 Tail Feathers and 7/8 Tail Feathers dies, the Mint finally settled down to the anatomically correct 7 Tail Feathers version. However, two additional problems required a fix. One was the eagle's breast, which appeared unnaturally flat on the earliest 1878 Morgan dollars. The other was the top fletch in the bundle of arrows on the reverse; early versions had a parallel fletch that was out of perspective with the way the arrows were aligned. These two problems were fixed on the reverse dies used in the final part of 1878 - the eagle's breast became rounded and the top fletch was slanted. The dies became known as the "Reverse of 1879" to differentiate them from earlier ones. Thus, the 1878 7 Tail Feathers can be found with either a Reverse of 1878 or a Reverse of 1879!


























