A mysterious Mercedes test mule has been spied in Germany.

Snapped wearing a Mercedes-AMG S63 Coupe body, the mule is cleverly disguised to hide in plain sight. While it's almost a dead ringer for the road-going model, a closer inspection reveals the mule has a significantly shorter wheelbase than the S Class Coupe. Nothing is official but not one of our spy photographers estimates the mule is approximately four to six inches shorter than the S Class Coupe with most of the room removed from behind the doors.

This suggests the mule is designed for a sporty and luxurious coupe or convertible with a relatively short wheelbase. Mercedes isn't talking but the car is most likely an early mule for the next-generation SL.

Little is known about the next SL but it is expected to be introduced in 2019 or 2020. Previous reports have suggested the company is developing an all-new platform for the model and it will also be shared with the next-generation SLC. The SL is also rumored to eschew its retractable hardtop for a more conventional canvas top.

Engine options remain unconfirmed but early reports suggest the car could have a 3.0-liter inline six-cylinder engine that has outputs of 365 horsepower and 435 hp.