All 28mm 1:2 Nikkors are very closely related optically. This type started with the Nikkor-N 28/2 in 1970. In its day it was a a very advanced fast wide-angle lens with 9 elements in 8 groups, fully multicoated on all surfaces (second after the 35/1.4), and with floating elements (third after 24/2.8 and 35/1.4).
The original style lens had an all-metal barrel with scalloped focus ring. The barrel was upgraded to the modern style with rubber focus ring at the end of 1975. The optics were refined at that time, probably by using improved glass materials and adjusting the curvature and spacing of some elements, but the overall optics remain basically the same as before. It was upgraded to AI in 1977 with no further changes.
In 1981 it was upgraded to AIS. The close focus distance was reduced from 0.3m to 0.25m, so the AIS is preferred if you like closeups (but if you like wide closeups the AIS 28/2.8 is even better - it focuses even closer and performs better at close range, although perhaps not as good at far distances). I'm not sure if Nikon used the same optics and simply extended the focus range, or if the optics were tweaked again. The AIS 28/2 remains an 9-element 8-group design in a very similar (if not the same) configuration as before. Some reviewers say the AI version is better, maybe the AIS sacrificed some infinity performance for improved at close range, but I've not seen any direct comparisons to support this and it could easily be a case of sample variation.
The total focus throw is the same for AI and AIS versions (120°) but the AIS obviously squeezes in a greater range so the focus throw at far and medium distances is less than the AI equivalent.
The aperture blades also changed, the AI and earlier versions had curved aperture blades so OOF blurs had a nicely rounded (but not fully circular) look. The AIS has straighter aperture blades so the blurs are more angular 7-sided polygons.
Late AIS 28/2 have the newer Super Integrated Coating (SIC), from about no 825xxx to the end of production. This coating is a light yellow-greenish color. Earlier Nikon Integrated Coatings (NIC) coatings were deep red/green/blue depending on the viewing angle. The AIS 28/2.8 had a similar change, compare the coatings. NIC:
SIC:
I don't know how much difference the newer coating makes, the older coatings were already very good and the 28/2 is known for being resistant to flare.