What Mercedes Model Is Hiding Under the Mock-Up Bodywork of This S63 Coupe?

Over the past two decades, the German powerhouse known as Mercedes-Benz gave the world a plethora of elegant sedans and neat high-performance machines. The development mule caught by our spy photographers a short while ago leaves us perplexed, though, chiefly because nobody knows what it is. From the looks of it, this appears to be your typical