Plus distribution box!!!!

Hello.

I've got a problem with the plus distribution can, well, better off. I looked for the plus distribution can today and in the can everything was top.
But a cable next to it, I suppose that's mass going to the engine block, the contact point looked much worse.
So I tried to unscrew the bolt. I sprayed rust solvers on the screw and carefully tried to release the screw. But it came as it had to come. Zack, and off was screw!!
Now I have a problem, you don't see exactly whether it was a screw or a standing bolt .... at least not me!!
If it was a screw, can you drill it safely? Of course I can't drive either ?!
Sorry, picture's on the phone.

Welcome
Andi