There are three main circuits on a GM starter solenoid. The biggun the battery connects to is the main power lug. When the solenoid contactor is made, power is fed to the starter windows.
Now, the two you are wondering about - the R & S. Take an ohm meter and measure resistance between the biggun to one of the little ones.
You should find the 'S' terminal has a few ohms resistance. I'll guess 8. The resistance is the coil winding that goes to the purple start wire. The 'R' terminal feeds 12volt power to the breaker point ignition during cranking. I don't use it but it seems I read here the wire was yellow.
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