Two totally different 12 volt starters were used on the 235 six cylinder Chevrolet light trucks during 1955-1959. They attach to different bellhousings and are not interchangeable.
As shown in the photos, the Hydramatic transmission starter has three bolt holes for securing it to the bellhousing. A solenoid on top reacts to the drivers key switch in the dash.
The starter for the 3 and 4 speed transmission has a top mounted foot start switch. It attaches to its bellhousing with two bolts