No help. There are many Mary Armistead's in the Elizabeth City, Va. Some name Westwood Armistead as the father but the mother Mary Tabb's birth date is listed as 1734. I can't say what the proplem is but I've seen many reports that Westwood did mary a Tabb. Robert Armistead also had a mary Armistead but I've looked at wills that mention a son-in-law of Joseph Selden and the gen of the Selden family name his wife as Mary Armistead daughter of Robert and Ann.
I'm certain that this is the Armistead family but I can't find the connection.