Without seeing it in person, it looks suspicious how the signatures are identical in every way, with little variation. Even the swish under the signature seem to begin and end in exactly the same place!

Make a scan or photocopy of the signatures and make them the same size. Then lay one on top of the other. If they match identically (keep in mind there may be a very minute variation if the paper got moved when "signing"), there's your answer.

Here's a site with proof hopefully!

Bought in book store looked it up looked like his signature!