to me the only thing interesting about the yearbook is whether the handwriting therein matches the handwriting on the ransom note....
Really?
Handwriting can be an affectation. If you're trying to disguise your identity...If you're pretending to be someone else whose writing you are trying to replicate...if you're a young person writing in a yearbook you often embellish your writing or choose an unusual style for it, due to knowing it's for posterity and you're trying to make a big impression.
Through the years some people play around with their writing and end up with quite a few changes. Some people don't do that. It depends on the person and the situation.