The part didn't jump out at me, and I didn't find a similar part on any of the B&Ws I have here.
With that said, my initial thought was that it's a Zip drive blanking plate. All the ones I have here at home(plus, I think, my one at work) have Zip drives so if that's the case I couldn't find one because I've never had one
In addition, the design is different from the blanking plate used on later G4 graphite cases. The cases on those are effectively identical to the ones on the B&W G3.
That will be where the extra screw went. Think it has a dead board anyway as it randomly boots some times and others it doesn't. Seems to happen if I put any pressure on the board I.e closing the case or plugging in the ide cable. New board on the way so hopefully that solves it. Thanks everyone