You have a solenoid-operated starter from either a passenger car or a later-model truck after the foot-stomp starter was discontinued. A big circular electromagnet mounted on top of the starter motor pulls a spring-loaded plunger connected to the pinion gear on the starter shaft into engagement with the flywheel gear, then a switch at the end of the plunger's stroke is made to run the starter motor. It's possible the starter brushes have a bad connection that prevents one of the solenoid windings from operating normally and pulling the gear into place, or wear and/or poor adjustment of the parts will let the armature begin to turn before the gear engages fuly.
Pull the starter off and clean it thoroughly, and look for any signs of excessive wear in the linkage. If there's nothing obvious wrong there, take the back plate off and look at the condition of the brushes. Have fun!
Jerry
