Whats the difference between 55-57 and 58-59 Wiper Motors?

[Maximus]

Looking at wiper parts...
the catalogs have motors for 1955-57 and 1958-59, what changed?
I originally thought maybe 58-59 came standard with an electric motor instead of a vacuum, but why would this matter in buying a new wiper motor? so i thought maybe the arms changed...
the reason I need clarification is I have a 1955 cab, but pulled the wiper arms out of a 1959 cab. I am assuming I buy the 1958-59 motor but before I drop nearly $200 I wanted some info behind this topic...

Thanks!!