There appears to have been several members of the Postlethwaite family that moved to Natchez MS around the same time.
Samuel Postlethwaite, Jr. b. 1772 Carlisle PA Parents Samueland Matilda Rose Postlethwaite.Moved to Lexington, KY 1790 with brothers Samuel and Joseph. m. Ann Dunbar 3/9/1805 Natchez MS d. 1824 Natchez MS
Sarah Postlethwaite b. 1763 Carlisle PA Parents Samuel and Matilda Rose Postlethwaite m. 1) John Duncan who died in a duel 1793 2) Ephriam Blaine Sarah's son, Dr. Stephen Duncan practiced in Natchez MS
Henry Postlethwaite b. 10/28/1778 Carlisle PA Parents Samuel and Matilda Rose Postlethwaite m. 6/10/1804 Elizabeth Mogran d. 8/27/1823 Natchez MS
Source:"A Rainbow of Families" by Emile Pragoff, Jr.