According to The Complete Nikon System the change to the size of the rear element was with the AI lens in January of 1977. The rear element is not very much larger than the one on the earlier 'K' lens but it is larger. The AIS had the same formula but a shorter focus throw and slightly different aperture click stop spacing. In addition to my two AI models I have three other 28/3.5 Nikkors. One is actually a CM lens. The older lenses are also good but have less even illumination in the corners at or near wide open. Some people assume that if a lens has the same maximum aperture and focal length, it must be just a cosmetically different lens. That's not always the case. The 35/2.8 'K' and the first version of the 35/2.8 AI were different from the earlier 35/2.8s (6 elements instead of 7) and also different from the late AI and AIS (5 elements instead of 6). The six element version is better than all of the earlier models and also better than the later 5 element lenses. If you only shot at f/8 or f/11 the differences between some lenses would not be as apparent but at or near wide open the differences show.