Mint mark: If none can be found it is (Philadelphia Mint), but many have the mint marks D (Denver Mint - 1921 only), S (San fransisco Mint), O (New Orleans Mint).
Notes: The 1878 8TF Morgan Dollar was the first type struck for circulation. It was these coins of which the first 12 were pulled by hand, after being struck by the die pair, placed in a special wrapper and given to the president and secretary. I am sure, as was often the case, that certain mint employees and other government dignitaries received the remaining 10.
The 8 tail feathers was considered a design error and was later modified and modifications of the dies resulted in a few VAMs or varieties. The 1878 8TF is known for these Top 100 VAMs: VAM-5, VAM-9, VAM-9a, VAM-14.11, VAM-15, and VAM-23. You can see these VAMs here Top 100
This date and mint are not rare but highly desirable and hold a decent premium when found in mint state and begin to become costly as grade reaches MS63.