A good starting place would be the biographical notes on Mary Ann Newcomb
(1809-94) and John Davis or Davies (1810?-85) in Huck Finn and Tom Sawyer
Among the Indians and Other Unfinished Stories, ed. Dahlia Armon and Walter
Blair, Mark Twain Library, University of California Press, 1989, pages 317
and 338-9. The notes indicate that she was born in Vermont and educated in
the East before moving to Missouri in 1839. In the 1840s, she married John
Davis (which he later changed to Davies), a Welsh teamster who later opened
a store selling "books and fancy goods" in Hannibal.
The biographical note on Mary Ann Newcomb cites a newspaper article from
the Hannibal Evening Courier-Post, 6 March 1935, 12B, which gives more
detailed information about her (although it is incorrect about her date of
marriage to John Davies):
Birthdate: 11 November 1809, Thetford, Vermont.
Parents: "John and Mary Curtis Newcomb, who had six sons, four of whom
entered the ministry, and three daughters. . . . Her father served in the
Revolutionary War. . . . Her mother was of the old Massachusetts Curtis
Education: "Mary Ann Newcomb was educated at Fredonia, New York, and
Descendants: The article mentions her daughter, Ruth Davies Watson, and
quotes two of her granddaughters, Miss Harriet Newcomb Watson, and Miss
Anna B. Watson, who in 1935 lived at 1328 Grace Street, Hannibal.
I hope this is of some use to you.
Mark Twain Project