On the 1870 census a Margaret Lee (b. 1836-1837) is living with her father, John Loudon and her son, nine year old William. It seems likely Mary Ann was Margaret.
"United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M6WS-KQ5 : accessed 2 April 2016), John Louden, Illinois, United States; citing p. 48, family 481, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 545,730.
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