My guess is its a FM. DOC appears to be far more of an active club than one that records and archives the actions of their members (if it is a DOC project in the first place). Nice to know there are still mysteries out there. FM = Frigging Mystery
The common method now are mileage markers that can last a long time. Installed at trailheads, shelters, or trail junctions
I remember a mileage sign about 2-3 miles NOBO from Iron Mtn Gap
''Tennessee Viking'
Mountains to Sea Trail Hiker & Maintainer
Former TEHCC (AT) Maintainer