I have the Chester County Heritage Book. It has several write ups on the Knox family. The writeup for John Knox and Elizabeth Martin Knox lists a son named John, Jr, b. 1816, married Martha Green Johnson abt 1841. Would this be your John Knox? Has a writeup on his grandfather and great-grandfather also, but does not list his children. Cannot vouch for the accuracy of it as I have not checked it. Will be glad to scan and send if you want it. Email me at Nachalink at yahoo dot com.