A direct Ai upgrade from the previous K model. The Ai model only stops down to f16 instead of f22, possibly because the aperture range was too large for early Ai cameras. Nikon Ai conversion kits for older models have the same reduction in aperture range. A few have serial numbers belonging to the K series and were probably made using up older parts.
400001 < 400006 – 402261 >
< 402897 – 403452 >
< 403991 – 416628 >
from 405xxx the aperture coupling prong made of pressed sheet metal.