This is a twin post to the previous one. The 906 was the first Piech-inspired Porsche and this was the last one, although he had nothing to do with it personally. (He can certainly take some credit for Audi's Quattro rally cars, and their DTM cars, and the R series of WEC cars, if only as the executive who approved, and probably inspired, those programs.)
An intentional loophole was created for this car in the 1981 LeMans regulations. The organizers were transitioning from regulations for Group 6 roadsters to Group C coupes (to take effect in 1982), and they needed some field-fillers. A one-year-only "sports" category was created for coupes provided they had an opening in the roof. The Kremer brothers, who had been very successful building their own Porsche 935 clones for the Group 5 "silhouette" class, obtained Porsche factory support in building the car. In return they provided feedback on closed car aerodynamics for the 956 Group C car already on Porsche's drawing boards. The opening in the roof was a rectangular hole through which the driver could see the center-mounted exterior mirror--just as on some privately-entered 917's ten years before.
The 917K81 (K for Kremer, not Kurz, and 81 for its competition year) was unsuccessful, despite a driver lineup led by Porsche star Bob Wollek. It retired at 23% distance with a broken engine mount, or 30% distance with an oil leak caused by an off-course excursion, according to which account you wish to believe. Nevertheless, it was said to be a fan favorite (probably for reasons of nostalgia and its non-turbo exhaust note). It was similarly unsuccessful in the few other FIA events it entered in 1981. If you want a more detailed description, here's a link to a good 962.com piece: