This summit is the old Horseshoe Lookout Site. It had a primitive platform on a rock pinnacle from 1921-1961. A standard L-4 structure, 1962-1983. Both structures succumbed to wildfires and were not rebuilt. Concrete blocks from the L-4 lookout remain. The access road, USFS #588 is 4 miles off FS #500.
Indian Post Office is an intriguing name given to a rounded high point qualifying as a SOTA summit along the Nee Me Poo, a trail of the Nez Perce and the Lewis and Clark trail. I searched the few vertical rock outcrops looking for what I thought might be pictographs or petroglyphs. I found neither. I did see a large rock cairn linked to folklore.
This summit is just off the Lolo Motorway that roughly follows along the Nee Me Poo Trail of the Nez Perce and the Lewis and Clark Trail. The Lolo Motorway, USFS #500 is a narrow mostly ridgetop road built by the Civilian Conservation Corps in the 1930s. It provides access to over a dozen SOTA summits along its route that are mostly a short hike.