I know of no documentation for a Mary Williams in the Box family of Laurens SC.The name is listed in many Box family trees but recent discoveries of documentation suggests otherwise. Keep in mind there were a number of men named Robert Box.On July 4, 1755, a Robert Boxand a James Williams both made petitions for land grants in South Carolina.The Memorial (from SC Archives) of Robert Box Junr indicates he received a land grant on Raborns Creek in 1755 about the same time as James Williams.After James Williams died in 1782, his wife, Hester made an oath that on his death bed, James Williams, in an oral will, gave the land on which he lived to his son-in-law, Robert Box,but there was no indication of the name of their daughter.On April 25, 1787, a Mary Box, widow of Robert Box, received the Revolutionary Indents for Robert Box and it has been assumed the deceased Robert Box was the Robert Box of Raborns Creek and therefore his wife would have been Mary Williams.However, there is documentation now known that suggests Robert Box of Raborns Creek was still alive in 1795 in Pendleton County, SC.I hedge on this because while the deed in 1795 indicates he made his mark with an "R" the same as did Robert Box, Junr. in 1755, his name was given only as Robert Box.
Averilla Box definitely seems to be a daughter of Robert Box, Junr. of Raborns Creek but Josiah, Stephen and John Morris Box are doubtful.It seems likely he would have had a son named Robert but documentation of that has not been found as yet. Edward Box, born 1755, Henry, Wiseman, and Jacob Box areprobably brothers of Averilla.
The connections of the Box family of Laurens Co., SC and the Box families of AL, TN, MS, AR and TX (and so on) are definitely there but are too complex to cover in this reply.