Well, it's not as though you could hope to find a piece of paper with a note "1627 BC" written in ball point pen. My father once tried that. He had an antique clock he'd repaired. Then, as a joke, he wrote, "owned by G. Washington" inside in modern pencil. He used to point it out to people who had no idea that a graphite pencil had not yet been invented back then.

On a serious note I have a question about this whole controversy that does not hinge on the dating. The article says that some of the cultures could not have coexisted because of a hundred year difference. Isn't that an awful short time for a culture to rise and fall?