Hi Holly I have done some work on this Lock family in the past. 1870 Lincoln County Tennessee census Page 431b Line 26 225/225 Nancy Lock, White Female, age 18, b. Tennessee Jane? Lock, age 17, b. Tennessee John Lock, age 15, b. Tennessee Jacob Lock, age 11, b. Tennessee H. M. Lock, Male, age 6, b. Tennessee Rufus South, age 5, b. Tennessee Ann South, age 4, b. Tennessee Andrew South, age 2, b. Tennessee E. H. South, Female, age 23, b. Tennessee
I went back and looked at this census again. The name appears to be South on the census, but after reading your post, it might be possibly Smith like you say. But it looks like South to me lol. I will take your word that they are Smith's not South's.
The Mullins family is no doubt related to the Lock family. There were several Lock/Mullins marriages. But if the Smith's are related, I am not aware how just yet.
This Lock family may have ties to Mercer County Kentucky 1790's