I don't think thats the actual name of it, I forget what rudes call em, its a silver cover on the block, about the size of a quarter. usually one on the side, and one on top. The number printed on it, is the serial number.
Replace the retainers, part number 315232. They are the keyhole-shaped plastic pieces that the needles go through. Of course you know your linkage between them is missing, right? But the linkage is not realy necessary