Everything has been more or less covered already, but it doesn't hurt to bring all the points together:
The older "New Nikkor" K 50/1.4 (K1) has optics which are based on the older Nikkor-S and Nikkor-S.C. The optics are not identical, the spacing and possibly the curvature and glass materials were slightly changed, probably to improve performance at close range as the K1 can focus to 0.45m compared to 0.6m for the older models. Pictures here:
http://www.destoutz.ch/lens_50mm_f1.4_2797729.html (actually Richard de Stoutz states this version focuses to 2 feet/0.6m so I need to check that...)
In January 1976 the K1 version was replaced by an all-new design (K2) which is much more compact before, reflecting the trend at the time towards smaller lenses. It focuses to 0.45m/1.5 feet. It is one of the first lenses to have the DOF scale on the chrome ring, a design feature which would become common during the AI and AIS series. Pictures here:
http://www.destoutz.ch/lens_50mm_f1.4_3777777.htmlIt was upgraded to AI in 1977 with no other changes ... they did change the rubber grip slightly, the AI rubber is less finely knurled and seems a dull grey instead of glossy black. Like the K2 version it has 5 screws in the lens mount, is engraved LENS MADE IN JAPAN in white, and has a black dash for the focus/aperture index. Serial numbers start with 3940001. We then have a number of minor changes...
At no 3980001 we get a slight change, the prong at the back which protects the rear lens protector is shorter, which means it can fit the TC-16 teleconverter. I had a feeling the overall length of the lens stayed the same which means the front grew in length, and I wonder if that reflects a minor change to the optics?
Between roughly 4046xxx and 4052xxx, the original dash index line gets replaced by a black dot, which is easier to read.
Around 4092xxx - 4095xxx, the engraving LENS MADE IN JAPAN in white is replaced by MADE IN JAPAN with no color.
Around 4125xxx - 4127xxx the number of screws in the mount reduces from 5 to 3. This and the above change occurred in many AI Nikkors, probably to reduce cost or production.
Between 4264xxx - 4269xxx the aperture ring now has two small screws on the side, so the design of the aperture linkage changed.
Production then continued more or less unchanged to the end of production at no 485xxxx.
The AI 50/1.4 was upgraded to AI-S in 1981, AF in 1986, AF (new) in 1991 and AF-D in 1995. As far as I know the optics did not change although I can't discount the possibility the optics were refined slightly along the way. All versions have NIC coating although they did change a little over the years, and the later AF-D lenses have the newer SIC coating.
