I really suspect this belonged to kid and he added a signature. It looks like he was going to add an inscription since there is "Best Wishes To" underneath.
Either someone tried to pen his early autograph or it's simply a non-malicious forgery. Whoever penned it, spelled the last name wrong. Looks like it is spelled "Mantel."
In the new picture you can see where the pens ink ran out after writing 'Jim'. There is nothing else written in the entire book. Most kids mark up a book and even write their own name inside. It just seems real strange.