"The Grand Central Mill: This rock wall was once the back wall of a silver processing stamp mill. This was one of 7 mills along the San Pedro that supported the 50 mines in the Tombstone Hills. The ore-bearing rock from the mines was loaded onto wagons and hauled to the mills by mule." from plaque at the site
You reach these ruins by walking on the San Pedro Trail out of Fairbank. An easy, flat trail that's even wheelchair accessible.
The trail itself is the old mule wagon road.
Highly recommended