This is a long shot but here goes. I am also researching Hartzog.I noticed the name Sylvester tied in with the name you are researching.
Searching information on Samuel Hartzog who Married Hope Heckathorn.Children we know are : Helen Mae, Leona Gertrude, Leslie Ervin, Violet June, Donna Rae, Roy Samuel.We have lost touch on this brand of the family ...now Hope Heckathorn's Grandfather was Sylvester Sage Gillett.I have Hope's birthdate July 2,1904 Winterfield township, Michigan married July 1922 and a death date of March 30, 1978 in Cadillac, Michigan. I thought perhaps this could be a decendant of our Roy Samuel Hartzog.Since you did not list birthdates for the names you are research I thought it was worth a note. If you believe that our lines could tie together please contact me a email@example.com or leave message in forum.Thank you