Hmmm, interesting. I always thought the 45P was made by Nikon. It is a unique lens - pity they didn't make more like it.
I have seen the Nikon branded AIS 35-70/3.5-4.8 and 70-210/4.5-5.6 which are made by Cosina. Apart from the rather "Series-E" type build, the giveaway is the number of aperture blades - 6 for the 35-70 and 8 for the 70-210 - which is not Nikon style at all. Nikon last used 6 aperture blades with the AI 50/2 and 55/3.5 micro until the late 1970s, and never made SLR lenses with 8 blades.
I'm not sure about the 45GN - this lens is very different by virtue of the GN mechanism, for example the focus ring turns the opposite way from other Nikkors and runs on a cam instead of a helix. Wouldn't that explain the different engineering? Or are there other differences - choice of materials, style of construction - which point to a different manufacturer? The hood is unique too, but even if it was made by a third party, surely it would be designed by Nikon. I don't have any hard evidence either way, but always assumed it was made by Nikon.