According to the book "Howard County Cemetery Records" compiled by Louise Coutts and Karen Boggs, Dewitt C. Miller was born 3-9-1836 and died 12-5-1903. He and Margaret are buried at Smiths Chapel Cemetery. This is listed as Smith Chapel on findagrave. Since Dewitt died in 1903, he died before death certificates were posted by the State of Missouri. These were started in 1910, which is the year Margaret died.
I hope this helps.
Harold Kerr, President
Howard County MO Genealogical Society