What is the difference between the rear rubber coupler that Mercedes dealers sell and the aftermarket ones that are available for less than half as much?

The aftermarket ones are Lemforder or Febi. The one at the dealer did not have any marking that indicated manufacturer. I did note that ones at the dealer appeared to have 12mm bolts whereas the ones on my 124s have 10mm bolts; I think the bolt size will not affect application.

The updated part number I used is 1294100015.

Edited:
From photos on Fastlane website I think the one at dealer was a Lemforder.