Nick - Kevin's info is correct that you use the number that is at the bottom of the application list (years and models) as described.
I did a little research and below is a screenshot from little later 1957 Chevy Master Parts Book with the parts for your stomp starter. You only mentioned not having the link, but I added arrows to all the parts that should be there just in case.
All the parts indicated fit the same years and truck models indicated in the screenshot and actual go up through 1959 trucks. Should be relatively easy to find either by posting an ad in the Swap Meet or searching the web using the GM #'s.
