I think my third cousin, Jason, found
Mary Dunn's younger brother.
He worked off the only clues I had about this photo:

I've been told the man with Mary in the undated (and truly weird looking) photo was Bob Dunn, who came to visit her from Texas.
As another blogger commented in the original entry, they look as if they posed in front of a bedsheet.
And then, there's that nail sticking out.
And the look on Mary's face.
She doesn't look terribly pleased to be there.
So Jason started looking for a Bob, or Robert, Dunn.
And he found one in the 1880 census, living three farms away from Mary and her second husband, David Andrew Williams.

So he tracked Robert J Dunn.
And found him living next in Hunt County, TX.
Robert James Dunn was born 3 Dec 1853 in Georgia. Like Mary.
He died 15 Dec 1926 in Hunt County, TX.
Jason and I cannot find his death certificate, although we have found death certificates for the same period of reported time, 1890-1976, for several of his at least 25 children by two different wives. (More about that later.)
We also cannot find his parents, or find him as a little kid in census records.
Like Mary.
Unless you count the church record, which Jason found at the Arkansas History Commission.
And which we believe shows Mary (Dunn) Callaway, her brother James Dunn, and sister, Martha Dunn, joining Bethel Union Baptist Church by experience in August 1867. (That would have been right after Mary's marriage to Mace Callaway, whose mother was already a member of Bethel Union, back in 1863, and before Martha Dunn married
David Andrew Williams on 27 Jun 1869 in Clark County.)

Robert James Dunn married Sarah A "Sally" Hickman on 10 Jul 1873 in Clark County, AR.
According to census records, draft registrations and death certificates, their first 10 children were born in Clark County, including Ella in December 1892. Mollie and Charlie Benford Dunn were born in Hunt County, TX.
However, the 1900 census in Hunt County said that Sally was the mother of 15 children, 14 of whom were living at the time of the census, so I've not yet found three little kiddos.
When Robert James Dunn turned up in the 1910 census without Sally, I thought I found another daughter, Bettie, and that Sally had died.
Then I remembered Jason's email. Robert had remarried on 18 May 1901 to Bettie Dorella Wofford. There was a congratulatory article in the
Commerce Journal:

"Parental inteference (sic)?" I looked more closely at the census.
47 year-old Robert Dunn married the 15 1/2 year-old daughter of one of his neighbors. "Prosperous farmer" or not, that couldn't have made them happy.
Especially since the 1910 census reported that Bettie was the mother of 5 children, 4 of whom were living at the time of the census.
And then I found Sally Dunn. Living with her son, William Oliver Dunn, and his wife Annie Anderson, in Mitchell County, TX.
Bob Dunn's third son was older than his new wife.
If Robert James Dunn is the Bob Dunn in the photo above, I have to wonder...
Did his marital hijinks have anything to do with the look on his sister's face?
Circumstantially speaking, I think we got our man.
And as Jason said, maybe together, we'll figure it out one day.
I'm going down to Clark County to nose around - real soon.