It is a reverse of a 1878 and a reverse of a 1879. The change in design. There is other cool varieties for this year but I like this one the most.
1878 is a fun year for Morgan dollars. The 8TF went into production before the mint was ready, but they needed to coin all that silver, anyway. Lots of crazy stuff on those dies. Two weeks into production they started phasing in the 7TF reverse of 78, then later in the year the reverse of 79, which stayed pretty much unchanged until 1904.