The factory M/b shocks do not come with the rubber isolators for the top portion of the shocks and you may bolt the new shock in from the box and discover that that the shock makes a thumping noise right off the bat, then you realize something is wrong and you discover that you have to transfer the top isolator bushing from the old shock to the new.
This is the rubber seal at the very top of the old shock that rests on the underside of the vehicle body while shock is in place.