BACKSIDE--91 TO CUMBERLAND (National Road PA)
Iron markers were posted every mile along the National Road, showing the distance from Cumberland MD. Hence it became known as the "Cumberland Road." Markers have two sides facing the road and are hollow in back. (Some replacement markers are of fiberglass.)
A Texas-New Mexico state line marker, installed much later of course, uses the same general principle. It appears on several images in the main Route 66 gallery.