Board Thread:Naruto Discussions/@comment-25122992-20170201160936/@comment-1512913-20170201202250

I agree with killman. Most likely the entire tablet can be read, the doujutsu just makes you understand the text. So in theory, the tablet could probably be deciphered without any doujutsu, albeit with extreme difficulty.

As you say, probably written in language or code nobody knows. Which would also explain how black zetsu could read and rewrite it without doujutsu, since he is from that era.