Six months after unveiling the Aventador, plans for a roadster version were leaked by the U.S. EPA after it included the model on a data sheet on its website. Lamborghini officially unveiled the Aventador J to the world at the 2012 Geneva Motor Show. The roofless and windowless concept car uses the same V12 engine as the standard Aventador, producing 700 hp through a lightweight seven-speed automated transmission.
The car has also been stripped of its air conditioning and radio units to save further weight, allowing it to weigh in at only 3,472 pounds (1,575 kg). The car presented at the Geneva show was the only unit to be produced, and was sold for US$2.8 million.
The J designation was thought to have come from Appendix J in the FIA rulebook that describes the technical specifications of race cars. However during an interview with designer Filippo Perini, it was revealed that the 'J' actually stands for Jota, in reference to 1970's one off Lamborghini Miura Jota which also conformed the FIA's Appendix J regulations. -wiki