If the bumpers are the same as an X300, they are secured, at the rear by 2 x 12m/m bolts, which go through an ally spacer held in a composit mounting block, the 'rivits' you see are the heads of the coach bolts that hold the mounting to the body, the nuts to these bolts are located in the boot. the steel bolt running through the ally spacer corrodes the spacer to the point that it no loger holds the bolt and the weight of the bumper causes it to drop, on mine one side went and when i had a look the other side was 90% through so I replaced both, not cheap £36.00 each from Jagspares Int. Hope this helps   Brian