It is the recreation of the famous Renntransporter or 'race transporter', used by Mercedes-Benz in the Fifties to carry the Grand Prix Racers to the tracks throughout Europe
Initiated by Mercedes-Benz race manager Alfred Neubauer, who wanted a fast option of getting race cars to the circuit or back to the factory if necessary, the Prototype-Department created a hybrid between a sports car and a truck. 
It started with a frame of a 300 S, extended it in the front and the rear, installed a 300 SL engine with direct fuel injection and 192 hp. For the cabin, components of the model 180 or Ponton were used - extended in width by several inches. The grille came from the SL model - only the rear glasses could not be found in any existing model and had to be especially manufactured. This was neccessary to allow a W 196 Streamline to fit on the Renntransporter. 
When it first appeared on the race tracks, the Renntransporter received more attention in the paddock than the actual Mercedes-Benz race cars. It did not fail to increase the image of Mercedes-Benz as a manufacturer of superb automobiles : 'If their trucks are already that fast and powerful, how much faster must be their other cars ?' But the days of the original Renntransporter were limited. After Mercedes-Benz withdrew from racing in 1955, it was still used for several years to promote the brand abroad. It even came briefly to the U.S., where it received a Max. Speed 105 mph painted on the rear fenders. Constant questions and rumors about the potential top speed were the reasons. In late 1957, it returned to The Works in Stuttgart and ended up at the Mercedes-Benz Museum. Due to limitations of the static of the higher floors in the museum it was denied a permanent place in the exhibition. Ten years later and in the state of disrepair it was scrapped. In 1993 an effort started to recreate what was so easily discarded some 25 years earlier. The result was first presented at Goodwood in 2001 and is expected to be part of all major events in the years to come.
The 1955 race car (no idea what the deal is on the new one) was numbered with 722 for the time of it's start on the 1955 Mille Miglia, when Sterling Moss won driving this particular race car. Stirling set the record that year for the fastest Mille Miglia, averaging about 100 mph, and no one since has beat that speed record in 52 years!
Dean Moon purchased a tribute transporter that is just as cool http://justacarguy.blogspot.com/2006/11/worlds-fastest-car-hauler-200-mph-had.html#links
