The Nikkor-P (P=Penta=5 elements) 300/4.5 was launched in mid 1964. It was replaced by the Nikkor-H 300/4.5 in 1969. The main difference is that the front element of the Nikkor-P was replaced by an acromat doublet to improve chromatic correction, resulting in better contrast and sharpness. Apart from that the rest of the optical design is almost identical. The extra element means the new lens is a Nikkor-H (H=Hexa=6 elements).
The doublet up from means the new lens is marginally longer and a little heavier, but the two versions are almost identical in appearance.
In 1975 the K/New Nikkor version was released with modern styling and multi-coating, but otherwise the same optics. It was upgraded to AI in 1977 and to AIS in 1981 without further optical changes (that I know of).
The AIS version has some other minor improvements however, the close focus limit was reduced from 4m to 3.5m, and the aperture now has 7 blades instead of 6.
All versions are beautifully made, but lack of ED glass means performance is not as good as modern lenses. The rather long focus distance (even of the AIS version) is also rather restrictive.