thanks for uploading photos Stripey, really detailed, excellent photography skills. :) now i know what you were talking about. this metal part connects battery to the switch , as the switch slides up and down, it actually connects from this metal plate to the motor, its common in little dc devices to use a motor casing ( the chassis) to act as one of the terminals. i can see that the battery negative terminal has a spring and blue wire connecting to one of the motor terminals, but the second battery positive terminal doesn’t have anything to connect to, thats where the metal part comes in using one of the right angle surfaces to act as a terminal for the battery, whilst the other part extends to connects to the sliding switch. what i don’t see is some little narrow slot next to the battery, where the part would sit securely making its way through the plastic to the switch. maybe it’s in the underside of the other cover ? have a look, and good luck.