Well I am coming very late to this 2020 thread, so hope someone might still find this of interest as member lyn had said it would be good to hear from the Summerton family! I am pretty sure that the family in the photo are Arthur Summerton (incidentally my grandmother's brother), who inherited the mill business from his father in 1912, his wife Lilian and his two twin sons Thomas and Robert, as MWS has observed. A third son was born in 1922. Don't know who the final young man is.
The picture must be from about 1923. The business went bankrupt in 1925: I don't know if it restarted, but it is interesting to see the directory entries above that imply that either the family or the business were still at 448 Park Road much later.
It is correct that Arthur's father Thomas Henry was born in Wisconsin. His father - another Thomas - went to the US in 1849, where he had a corn mill and business dealings with the Horlicks family. Both Thomas and his son Thomas Henry returned to Birmingham in the 1870s.
The family roots are in Oxhill, South East of Stratford-on-Avon, where they can be traced back to before 1600.
Tim