The Phaistos Disk!
Probably a fake. At least in this timeline! Or at least that's what they want you to think...
It's a ceramic disc inscribed on either side with a spiral of symbols in no known sensible language. Some people have claimed to have decoded it. Others say it's just a fake by a hack archaeologist. The museum that has it (along with another anomalous axehead) is refusing to let anyone test it, which is mighty suspicious, don't you think? There's an image at the link, which could make a nice reference if you're using this as a MacGuffin.