in the stock starter when you twist the key all the way to start it sends power to the relay/starter solenoid to the S terminal to engage the starter after you release the key the switch goes to the run position and cuts the power to the s terminal on the starter solenoid. so in order for you to have a run on condition on the early type starter either the key switch is stuck in the start position sending power to the starter solenoid. this is easy to check with the starter stuck running on pull the wire off the s position on the starter solenoid if the starter stops and disengages it was the ignition switch, if it stays on, then the starter solenoid is stuck on. its that simple on the electrical side. there could also be a mechanical issue inside the starter keeping it stuck in place. common method of failure is the solenoid first, the ignition switch from lack of use and then the mechanical failure of the Starter drive. bad gear or bad teeth on the flywheel can also cause issues. hard to tell looking from a computer screen.
