The old Ford type solenoids were marked "I" and "S" to indicate the use of the small terminals. The 2 large terminals were connected to the battery (+) and the starter lug. The 2 small ones: One marked "I" was for the ignition bypass. It put a full 12 volts to the points for starting the engine. When you released the key, the power to the points went back through the ballast resistor and lowered the voltage across the points to prolong their life. The "S" terminal was for "start" or to energize the solenoid to pull in and close the 2 large terminals to put battery power to the starter motor.