The green pointer should be on the roof of the Hildebrand-Beck Mill. The site is on the east side of Flint Creek about 3/4 mile north of US Hwy 412 (Cherokee Turnpike) and about 9 miles west of the Arkansas-line at Siloam Springs. The square roof you see is the remains of the 1890 Hildebrand-Beck mill on the same location as the original. The original was destroyed by flood in 1890 and rebuilt and enlarged.
This mill was powered by an undershot water wheel. The mill race is clearly visible coming in from the northeast. The grindstones came from France via New Orleans to Ft Gibson and then by wagon to the mill location. The mill was built by a Cherokee named Towers, then acquired by Joseph Hildebrand, operated by his son Stephen when the war started, later acquired by the Beck family -- these families were all connected through marriages. When the war began, "Old Man" Joseph Hildebrand lived just north of Webbers Falls where the Hildebrand Ford was located. I last visited the site about 5 years ago. The property is owned by a Beck descendant that lives nearby.
There is an article on the origins of the mill in the Chronicles of Oklahoma but I could not find it online. I may have it in a PDF file I can email to you if I can find it!
By the way, my ancestors are connected to the Becks and to Zeke Proctor through marriage. Zeke accidently shot Lucy Beck Kesterson at the mill in 1872 leading to his trial at Goingsnake Courthouse at which the Becks tried to capture/kill Proctor resulting in the "Goingsnake Massacre".