I don't have much information about Art Knackstedt Sr but the Carondelet Historical Society has info re: young Art [WWII] - Their contact info is 6303 Michigan Avenue, St. Louis, MO 63111 (314) 481-6303
info@carondelethistoricalsociety.org I found a site by accident that has info on the Roemer branch [George's wifes side] Not really linked to you but it gives some info on the Knackstedt side