The following Q&A is excerpted from Clifford Mishler’s Coins: Questions & Answers:
Q: I understand some of the silver dollars of 1878 have eight tail feathers on the eagle, while all subsequent Morgan dollars have only seven. Why was this change made?
A: The eight-feather type was made in error, as Morgan’s original patterns had seven. To correct this deviation from the approved design, it was necessary to make a new master die with seven feathers. At least one of the working dies that had been made with the eight-feather master die was directly altered by the engraver’s hand, thus creating a 7 over 8 tail feathers variety.
There are actually four distinctly different die varieties known for 1878 Philadelphia Mint issues: The eight-feather variety, the 7 over 8 feather variety, and the seven-feather version with both parallel and slanted arrow-feather treatments of the bundle of arrows held in the eagle’s claws. While only about 750,000 of the eight-feather variety were minted against a combined quantity of nearly 9.76 million for the other three varieties, all are of relatively equal rarity — except in the upper Mint State range, where the 7 over 8 and seven-feather with slanted arrow-feather varieties command premiums roughly double those for the eight-feather and seven-feather parallel arrow-feather varieties.