Thanks for your quick post ... I am a little confused please clarify if you have documentation that your Mary Davis (married George Chamblin) was a sister to Spencer Davis. Also I think you substituted the name "Nancy" for "Mary"?
You say that Spencer Davis married Nancy Furr. Can you tell me the source of this? What date do you have for this marriage? Perhaps this is an earlier Spencer Davis? The facts I have say that Spencer Davis born in Loudoun county about 1761 married Ann (Nancy)Wornall 1786. I do however beleive that Spencer's mother's maiden name was Furr, as I have documentation that his uncle was Enoch Furr.
My Nancy Davis Chamblin sister of Spencer Davis was indeed born 12/31/1767, but I am currious on your source date of 2/1840 for her death. Nancy Chamblin signed an affidavit in here home of Loudoun County on 2/28/1840. Elsewhere I have seen reference to this 2/1840 death date in Jefferson OH, but I find a Nanny Davis living on the 1850 Loudoun Co., VA census with an Enoch Davis, who I beleive is her brother.
Anyway Blah Blah Blah ... Bottom line is I am very interested in what you may have that ties Spencer to a father named Samuel Davis.