This is the correct gearbox used with the 914-6 GT race cars. They used a limited slip with either a 7:31 or 8:31 ring and pinion. Some 901 racing transmissions were later upgraded with a kit to aid in the cooling of the transmission as seen in one of the photos below. These transmissions were found not particularly easy to shift by many race car drivers as they required a "smooth-shifting-driver" (in other words, a "professional road-racing car driver. No drag-racing type shifting allowed here!). The 901 gearbox is often referred to as the "tail-shifting" transmission (the shifting mechanism is located at the rear, in the tail-housing).

Great reference page about the 901 and 915 Porsche transaxles http://www.blueskymotorsports.com/index.php?option=com_content&view=article&id=17&Itemid=6