With the old,pre-Ai Nikkor 55mm f/3.5 Micro Nikkor lens's manual diaphragm ring set to f/5.6, the "buckhorns" align with the Nikon tube's white mounting index dot. In the olden days of pre-Ai Nikkors and the F Photomic series of finders, the lens had to be set to f/5.6 for dot-to-dot alignment. While not necessary when adapting a Nikkor lens to a Canon body, it's still easiest to set the lens to f/5.6, just so the lens will mount perfectly onto the extension tube.