It's an AMG thing, and it's called a ducktail. In the 80s, AMG in Chicago had a local guy build and weld these ducktails onto some of their w124s. AMG had a bolt-on ducktail available, but Buxbaum (head of AMG of NA) wanted something better.
A few years ago, I owned a w124 AMG that was a hopped-up 6-cyl and it also had that rare welded AMG ducktail. Here's a pic:
