Theodor Tumbush had three sons, Henry, Bernard, and Herman, all of whom lived into adulthood and had sons of their own to carry on the family name. It appears that most of Henry's children chose to change the spelling of the name to 'Tumbush' - their birth records indicate the original spelling, while military, marriage, residential, and death records are most often without the 'c.' As far as I can tell from the records, the children and grandchildren of Bernard and Herman kept the original spelling.
If you're not one of my in-laws and have never heard the name 'Tumbusch' before, you're not alone. It's not a common name. The Worldnames Public Profiler maps surnames for 26 countries around the world using recent telephone directories and voter registries. It is not scientific or completely comprehensive, but it still gives you a good idea of relative popularity of a name in different places. In the U.S, the frequency per million (FPM) for the name Tumbusch is only 0.41 (The Tumbush spelling is 0.25). By comparison, my surname, Kowalski, which is basically the Polish equivalent of Smith, has a FPM of 81.56. And Tumbusch is actually MORE common in the U.S. than in Germany, where the FPM is only 0.09.
Tumbus(c)h family documents can be viewed and downloaded at this website: http://schroeder-tumbush.weebly.com/tumbush.html