Sorry I have not responded. My nephew reminded me of this post. This is the information I have on the Postlethwaites in my database:
Samuel Postlethwaite m. Matilda Rose -no dates. Children: Samuel and Henry.
Only info on Henry is death date of August 26, 1823 in Natchez, Adams County, MS
Samuel: b abt 1774 in Pennsylvania m. Ann Dunbar March 9, 1803 in Adams Co. Their children were: Eliza Dunbar, Amelia, Dinah Clark (Ann's mother's name), Margaret, Matilda Rose and William Dunbar Postlethwaite.
Eliza m Alfred Cochran 11/23/1831 Adams Co. Amelia m. George S. Mandeville12/29/1842 Adams Co. - They had one child: Duncan Linton Mandeville. Dinah Clark P. m. Charles Gleason 9/27/1852 Margaret- no information Matilda Roseb. 1807 m. George Potts 4/24/1824 d. 8/20/1860 - George b. 3/15/1802 in Philadelphia, PA, d. 9/15/1864 in N.Y.C., N.Y. - show 2 children- Mary (no info) and William d. age 5 William Dunbar Postlethwaite b 9/24/1810d. 3/13/1863 m. Susan Smith 2/2/1832 -- second marriage to Sophria Carter 8/2/1838no children listed for either marriage.
I have not verified all this information since it is not a direct lineage. Hope this helps someone.