A nuance that's forgotten about film-based photography was the endless challenge of film speed. We wanted to make prints with great resolution at large sizes, but to do that we needed to use film that was "slow" in reacting to light. You see, to make a proper exposure with film, enough photons have to strike enough molecules of light-sensitive silver bromide/halide (or color film dye layers). The finer the apparent "grain", or resolution, of the film, the more densely packed the granules of silver salts were and thus the greater amount of light required to expose each shot sufficiently.
Today's digital cameras largely ignore "speed ratings" as their electronic sensors can see quite well in even very low light. Light levels that are now "normal" for image-taking. But back in the day, a roll of 1936 Kodachrome transparency film had an ASA/ISO value of 10. To put it in perspective, shooting original Kodachrome meant exposures on a very bright, sunny day of 1/60th of a second shutter speed at f/5.6 lens apertures. Even a dull day meant putting the camera on a tripod. Sports? Not easy.
That's why your elder film camera users/collectors get so excited about "fast" prime lenses instead of great zoom lenses. For us, the extra one-half to one full stop of light gathering ability at minimum aperture was the difference between using a tripod or not. Old habits will die with us. Even though a 135mm, f/3.5 lens shot brilliant, clear images, we held out for a f/2.8 version to buy. These days f/3.5 or f/4 maximum openings on a lens is considered fast (enough).
So, OK, now let's get to Nikon here. Partly to help with the needs of professional photographers shooting in varying lighting conditions and party to show-off their manufacturing prowess, most film camera manufacturers - especially Nikon - offered a few very high speed (and coincidentally very expensive) versions of lenses in the most popular or useful focal lengths.
For "prime" lenses (what we called "normal") of around 50mm (1x magnification on a 35mm camera), you generally had two lens speeds from which to choose; something around an f/2.0 Nikkor, and a "faster" lens of f/1.4 Nikkor. Either lens took great images at shooting apertures of f/8 or so. Ah, but that extra light-gathering of the f/1.4 might get you a better exposed negative in lower light. Average consumers swallowed hard and bought the f/2.0, well-heeled consumers and pros opted for the f/1.4.
The other thing a lens with such a large minimum aperture offered was a razor thin depth of focus ability. When you focus a lens you're moving an area of sharpness forward and back from the camera's position. With a lens set to f/4, that area is decently large (maybe a few feet at distance). With an f/1.2 lens that area of focus is only a few inches wide when you shoot images closer-up; fabulous for shallow depth of field images in portraiture. Shallow as in you could have one eye in focus on someone's face, their other one blurred kind of effect. Today, you'd simply whip open Photoshop and add Gaussian blur to an area. Not back in 1965, it was all done live on film (or in the darkroom if you had one).
The Nikon 55mm f/1.2 of 1965 was a bit longer focal length "normal" lens. This was done to aid construction to accommodate the large mirror box of an SLR camera in those days. The lens didn't need to protrude into the mirror box as much. Optical formulas were worked out by hand, not computer then.
By 1977 Nikon had developed a 58mm version designed with special aspherical elements (more showing off). Marked the Noct-Nikkor (Noct meaning night) the lens improved the cancelling ability for the prominent flairing and coma effects typically produced by lenses of the f/1.2 variety when shot at full aperature. Of course, nowadays, we would WANT those in-camera defects.
The final 50mm AI variant came along in 1978 just as film speeds with tighter grain arrived. The days of the Nikon f/1.2 normal lens ended in that year. Too bad. They were marvels of optical construction, the likes of which we may never see again.