Hi, all! This is rather late-to-the-party post here, coming as it does 11 years later. But I just thought I'd mention that the Spiratone/Kenko lens was not, in fact, ever used in 2001: a Space Odyssey.
The HAL faceplate lens was a commercial lens, and a fisheye at that. But Nikon (Nippon Kogaku Nikkor to be more accurate to the time) 8mm f/8 lenses were actually used. This can be verified through examination of stills taken from the 4K release of the movie - you can't read the lettering around the barrel, but you can make out individual white blobs for each letter or groups of letters. That makes it possible to match the same lettering as seen on an actual Nikon 8mm f/8 lens. In addition, one behind the scenes photo clearly shows a lens with its cap installed, and it's a really unusual cap. Finally, the Kenko lenses were too small in diameter to make sense as a HAL lens, unless the plate were tiny.