Rattling noise in case, spare screw found.

I heard my phone rattling a bit today, so I popped it open to see what was loose. I found a stubby screw stuck to a black (magnetic?) bar underneath the battery, and looking at screw diagrams it looks like it's supposed to hold the left side of that black bar down. Unfortunately, the screw doesn't seem to fit in that hole (won't tighten down like a good fitting screw).

Any help with what to do? Thanks!