New OEM and direct from Germany! This is the critical thick rubber seal located at your fuel tank outlet screen just above your right rear axle. More often than not this seal is badly deteriorated due to diesel fuel contamination and age. This seal prevents fumes, moisture and road debris from entering your trunk.
When cleaning or changing your fuel tank screen it would be a good time to replace this seal. You do not need to remove the fuel tank but you will need to remove the four anchor bolts (two on each side of tank) to allow lifting the tank enough to slide the seal in place. Silicone or Goop should be used between the tank and the upper lip of the seal to help hold in place. Next time you are under your 123 inspect this seal.