Nikon AI kit #40 will only fit chrome barrel Nikkor-P 105/2.5 lenses with serial numbers between 234011 - 286451. Earlier lenses have the same optics and general appearance but the internal design of the aperture ring is different and it will not fit.
It will also not fit the later black barrel Nikkor-P 105/2.5 with serial numbers 407301 - 571564.
If you want to convert an early Nikkor-P 105/2.5 with focal length marked 10.5 cm and 9 aperture blades, the best option is to have it converted by Marcel Van Engen - see
https://magnimopus.com/services/. The advantage of his conversion is that it preserves the original appearance of the lens so does not degrade its value. I had my 10.5cm converted by him and he did an outstanding job.
Note that 10.5cm lenses with 9 aperture blades are very early - made between 1959 - 1960 and only about 4500 were made before switching to 6 blades. It may be difficult to find one. Such old lenses may also need cleaning and re-lubing. Also note, the first ones made up to serial no 123631 have a "long skirt" aperture ring - the aperture ring overhangs the mount more than later lenses. This extra overhang my need to be trimmed off to provide enough clearance to the camera even when AI converted - they don't even fit the Df.
An easier route is to get an AI 105/2.5 instead. It fits the D850 perfectly without modification. It has the later gauss optics which is a little sharper and performs better at close distances. It focuses closer (1m vs 1.2m) which can be useful. This lens is well suited for portraiture, the sharpness is good and rendering of backgrounds is smooth. It is multicoated so contrast and colours are better than the older Nikkor-P lenses (but some prefer the more muted rendering of the older lens). The AI 105/2.5 has 7 aperture blades which are slightly curved so when stopped down the opening is not a hard straight-edged polygon, the opening is more rounded (but not fully circular). Don't get the AI-S version - the optics are the same as the AI but the aperture blades are straight-edged and the focus throw is shorter - focusing is quicker but less precise.
