The well nut, (The rubber sleeve part.) fits into the frame. The screw is just one of the odd button head 5mm screws that hold on most of the bodywork. The well nut is just a press fit and relies on friction between it and the frame basically to enable you to do up the screw not very tightly.
Well nuts of this type are readily available, at least in Oz, at most fastener shops. They're nothing special.
The picture is deceiving as the well nut is in the wrong place.