I love those. I used to live in both NM and Arizona and found quite a few exploring on my own. But what I was really looking for was a specific individual who wrote, carved or otherwise scrawled his or her name into eternity.
All of Hammurabi’s (1700s BC) letters on clay tablets are written in the same hand. Some believe that Hammurabi himself wrote them. (Given the length of his reign and all.) His name of course appears in the tablets and might thus constitute an autograph.