I've found this sentence in Google Books:
Somewhat more permanent are small anodized metal “A.T.” diamonds that the original trailblazers nailed along the path in the 1930s and 1940s.
Am I right that "A.T." means "Appalachian Trail" here? The following sentence in Wikipedia made me think so:
The Appalachian National Scenic Trail, generally known as the Appalachian Trail or simply the A.T., is a marked hiking trail in the Eastern United States extending between Springer Mountain in Georgia and Mount Katahdin in Maine.