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I have slao noticed this same notation on records of my family. There is one gg-aunt and grandmother on my maternal side where she and all of her children are listed as "MU". At my g-grandfathers' funeral my mother told me of the cousins who attended from Mo. who looked white. They are descendants from that line. My aunt was the "Mu" but her mother was not. My grandfather gave us this information.
3 years ago on The Mulatto Factor in Black Family Genealogy