Because I could not believe it myself, I went and checked and it seems to be authentic. Link to a digital version of the book (the picture appears on page 30): https://kokusho.nijl.ac.jp/biblio/100357933/1?ln=ja
This is the earliest example of a cartoony open smiley mouth I've ever seen. It's pretty obvious it was make to look cute when this is what a hammerhead's mouth actually looks like:
Not open but a cartoony smile. I feel like it might be anything though, some notation to indicate its contents or whatever else, but we don't know and probably won't ever know.
Because I could not believe it myself, I went and checked and it seems to be authentic. Link to a digital version of the book (the picture appears on page 30): https://kokusho.nijl.ac.jp/biblio/100357933/1?ln=ja