With the introduction of The Nikon F, its first SLR, Nikon introduced the range of lenses to go with it. These collectively known now as Pre AI.
Apart from those lenses that required the mirror to be locked up before mounting, the telephoto's requiring a AU focussing mount, and the big cats, most had a meter coupling shoe ( rabbit ears ). The first meters were clip on selenium units that could be coupled to the shutter speed dial and to the lens by means of the shoe.
Pre AI lenses can be recognised by the solid shoe. When the range was progressivly updated to AI design the shoe was continued in order to ensure backwards compatabilty. AI/AIS lenses had slots in the shoe to let light through to a secondary smaller aperture engraving. This engraving was used to display the set aperture in the viewfinder via a prism.
Mounting of pre AI lenses on the early cameras required that the aperture first be set to f5.6. The lens was then mounted and twisted until it locked. The aperture ring was then turned through its minimum to maximum aperture range to index it with the meter. Oldies will remember that one.
With the introduction of AI ( auto indexing ) this was no longer necessary unless an AI lens was used on an old camera with a meter coupling prong.
Of the older manual range, E series lenses do not have a meter coupling shoe but
ARE AIS.
The shoe method of coupling a lens to a meter was later taken up by Mamiya with its M645 1000 S in the 1980's. Just as the Nikon F and F2 had seperate metering finders, Mamiya introduced a metering prism finder for its medium format SLR. This was before electronics in the shape of the Bronica ETR, invaded medium format, so a mechanical linkage was needed.
Pics attatched
1) The meter coupling prong.
2) Mounting a pre AI lens
3) Pre AI compatible lens mount
Don