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I'm a little confused by your line from Mary Anderson/Samuel Mosby. These are my g-g-g-grandparents, and they had 8 children as far as any records I can find: John L., Overton A., Joseph, Robert, Benjamin, Sarah Anne, Mary Samuella & Elizabeth Carr.
Thomasia Price Mosby was the daughter of Elizabeth Carr Mosby & John Marshall Price.
John Marshall Price's g-grandmother was Barbara Overton, and the Overton name was quite common after that. Those families married each other rather often, but I don't have records that explain the Overton name as a sibling of Elizabeth Carr Mosby.
Incidentally, my meager records show that at least two of those 8 lived in MS -- John L. Mosby & Benjamin Mosby who married the Woodson girl (I have her name as "Palica" which seems unreal! Must be a 19th century handwriting problem.)