There are three different varieties of 1878 Morgan Dollars which PCGS recognizes. We recognize the 1878 8TF, the 1878 7TF and the 1878 7TF with Reverse of 1879. These were the first Proof Morgan Dollars struck by the U.S. Mint. It's mintage is significantly smaller than most other dates.
However, there seems to be some disagreement on the mintage of the 1878 Proof Morgan Dollars. Some believe a total of about 500 examples were struck, while others believe as many as about 750 were struck. Regardless of the exact mintage figure, the coins appear to be scarcer than the 750 mintage figure, as fewer examples have been confirmed.