UPDATE: The lens has been, inexplicably, removed from sale.
One of Nikon's most esoteric and desireable lenses, the 13mm f/5.6, can be found on eBay currently. Designed by Ikuo Mori and boasting an angle of view of 118-degrees it was, at the time of introduction (1976), the widest rectilinear lens made for 35mm camera and an ideal lens for architecture. It was also one of Nikon's most expensive lenses, built to order only less than 400 were made in total. The lens for sale looks like the first version which has been factory modified for the AI mount. The second version starting at 175501 was made with a AI mount and the third version from serial number 175901 was modified for AIS.