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Looking at a T41 chassis, my best guess would be the threaded insert on the Ultrabay eject lever - the one where the security screw goes into. Do you have any dimensions?
edit: Can I change my vote?

How about one of the threaded inserts that goes to the palmrest? If somebody installed the wrong screw (long one instead of short), then that can liberate the "nut" from the plastic tower its mounted into. Those are about 3mm in diameter and will accept any one of the 2mm screws.