Nathan is listed here in Stout and Allied Families on Solomon's page:
https://archive.org/details/stoutalliedfamil00stou/page/116/mode/1up?q=solomon
Because of the Quaker connections, I looked in Abstracts of the Records of The Society of Friends in Indiana, but I don't see an entry for him in either volume. (I'm kind of perplexed as to why he isn't there.)
However, there is some information in this old newspaper:
"The mill had been built by Solomon and later operated by his son, Nathan Stout, her great-uncle. Nathan married Sophia Swengel and they were the grandparents of Rex Stout, who was born in Noblesville to John Wallace Stout and Lucetta Todhunter. Farmers brought grain from miles around and waited for it to be ground. When the mill, which had been sold to the Helt family years before, collapsed into the swollen creek in 1912 it was one of the long-remembered and much discussed incidents in the area, A severe winter with ice and snow, much like the past few months, resulted in early spring flooding; ice jams weakened the stone foundations of the mill. Miss Newsom, retired schoolteacher' and historian for the southeastern part of Bartholomew, County, had been told about the recently published biography of Rex Stout, who was of her generation) by a nephew, Carroll Newsom, who lives in Fairfield, not far from the Rex Stout home."
https://www.newspapers.com/newspage/138676413/
Unfortunately, it's historical and not contemporary. Also similarly (related to the same mill) at Indiana Gen Web:
http://ingenweb.org/injennings/pages/histories/helts%20mill.html
I'm sure with more research the contemporary sources can be found.
