Mercedes-AMG won’t admit it, but there’s a faster, lighter, altogether scarier AMG GT on the way, believed to be the sixth member of the brand’s Black Series, and here’s a first look at its development.

This AMG GT R mule was spotted testing at the Nurburgring with a few tell-tale differences to a normal production version – particularly around its back end.

Glued into the camouflaged rear hatch is lightweight polycarbonate in place of traditional glass. In the video above the window looks production-ready with demister grid lines wired into it. On top sits a normal AMG GT R spoiler with its adjustable blade dialled to a sharp angle for increased downforce.

Beneath the rear bumper is a modified diffuser and exhaust taken from the AMG GT4 racing car (below). It is similar in style to that of the normal GT R, though its central tailpipe has been deleted and a more conventional twin outlet system has been retained.

Further clues that this is an early Black Series prototype include the fitment of every optional performance-adder available for the regular GT R, including carbon-ceramic brakes (which were standard on the SLS Black Series), fixed-back bucket seats, and the Clubsport Track Package, which includes a rollover cage and four-point harnesses.

Elsewhere it’s standard GT R fare, though this car is unlikely to be testing with the regular 430kW/700Nm engine tune. AMG’s 4.0-litre twin-turbo V8 produces as much as 450kW/850Nm in the E63 S. A GT Black Series would almost certainly reclaim its flagship power status with at least 450kW, though perhaps with less torque than the top-billed E-Class.

Moers has told Wheels previously that “the chassis of the GT R is capable of a little more power output, but we’re not going to put an E63 engine into a potential Black Series, that’s not going to happen because an E-Class’ 4.0-litre V8 is a torque engine.”

In discussing the hypothetical addition to the GT line-up Moers put the focus on driving dynamics rather than outright grunt. Reducing weight with an even more extensive use of alternative materials (such as the acrylic rear window seen above) would go some way to achieving that, as would further exploitation of the GT R’s four-wheel steering.

“With the GT R we moved the bar so far. [Black Series] needs extra driving dynamics on top of the standard car,” said Moers.

Black Series versions typically arrive at the end of a model’s life cycle and are always based on a two-door product. An AMG GT Black Series seems ripe for picking. Speculation suggests it will go on sale in 2018, and that it could be shown as early as next month at the Frankfurt motor show as part of AMG’s 50th anniversary celebrations.