Wiring for a engine stand
FWIW. This is from a Delco-Remy catalog.
Similar to the Chrysler starter connections.
GM has their own terminology for some things. In this case "magnetic switch" is what they call an electromechanical relay. Others will call that style a "Ford type" although its also found on mopars in the very early 60s.
The relay is used when the push button is located some distance away from the starter.
This keeps wires connecting to the starter solenoid (Switch terminal and wires #1 and #2 in the drawing) short.