This is the engine shut-off solenoid. This is what replaced the push-pull cable ten years ago while we were in Key West. Yes, only one bolt is needed to hold it in place. If the mounting tab ever fails, there is another on the other side. This solenoid is just an over the counter item that I was given when I asked for a shutoff solenoid. It is generic. Most diesel engines use them except Yanmars. Grrr.
Run a wire from the hot side of the ignition switch (hot when key off) to a push button that you install in place of the pull cable. Make sure to use a waterproof cover in the cockpit. Then run the wire to the solenoid. You're done. It'll ground to the block.