Grounding post question.

[SierraScreech]

This is the grounding post on my truck. I am trying to separate the top portion from the bottom portion. It is definitely two pieces on the top, underneath is a nut or something which sandwiches it for mounting and I don't want to part it there because I cannot get at the nut to reinstall it so if that comes loose I'm screwed. Is it compressed on over it or is it threaded on? I've tried to back up the bottom piece with needle nose vice grips to try and break the thread loose to unscrew it but cannot get a good enough grip. I've tried a small prying tool to see if it can come loose but was unsuccessful as well. Anyone know if it's threaded on or not?

[SierraScreech]

Looks like a compression fit to me but am not 100% sure.

 

[Chevy sad]

Looks like a set screw on the side

[SierraScreech]

That's just a piece of metal shaving from the top or water drop. No set screw.