There are a "ton" of lightly circulated 1879-O and 1880-O dollars out there. Yours was probably one of those coins that someone decided to dip, or clean, to try and make it look better. It didn't work.
I always heard that the New Orleans mint released those dates in quantity and then recalled them days and weeks later. I know that there were solid date bags of lightly circulated 79-O and 80-O dollars in the Treasury hoards that were released in the early 1960's. (Same thing occurred with some 84-S bags too that had nothing but lightly circulated coins in them).
Bet that's where your coin originally came from.