Here is the bonus cache after completing the "Grant Wood Scenic Byway - Jones County" Adventure Lab. Clues to the final coordinates are revealed in the journal entries for the 5 adventure lab stops.
The final is another historic place to visit. It was constructed in 1887.
No need to do anything dangerous at the final. Stay on the 'safe' side of the fence!
March 11, 2023 Update: the area around GZ appears to be permanently fenced off. A new cache has been moved 40' to the NNW. The Adventure Lab, Geochecker and Hint have been updated.