I was just assuming that Sarah married a Thumma because Andrew Thumma is listed as a stepson of Alfred Barnhill on the 1880 census. I think your theory is very possible and that Andrew took his stepfather's surname. That 1870 census shows Alfred Barnhill as only 22, very young to have fathered Andrew b. in 1863. Something else occured to me. Perhaps Sarah Comp was not married to a Thumma and Andrew was born out of wedlock.