it is in rotoloni s book complete nikon rangefinder system .
it is both a rangefinder magnifier and a diopter adjustment .
it magnifys the rf spot and finder from the 75% of the standard nikon s to life size . the back part turns to provide diopter range for different eyesight corrections .
it is fairly rare and not many are seen .
my guess is that the 85 / 1.5 was used on a nikon s and the magnified rf spot allowed more accurate focus , and perhaps provided an approximate field of view for the 85 lens .
pm me with your email address and i will try to send a copy of the specs in roberts book .
the only decent garage sale find i had were a 5cm f2 shade and a 5cm 1.4 shade . the f2 shade was in the box . thats in 35 years of looking .