DUH, Is there a threaded stud just like the other 2 ? Is there a nut on it just like the other 2 ?
I'm trying to help.. I dont know if it is a required copper conductor, or a cover to keep the noise supression capasitor connector off of it, a spring steel fastener, or even a heat sink for that internal limiter.
Help me help you. Post pictures of your panel or something.