That lug at the arrow does lock/unlock the aperture ring, whereas the later MC lenses
have the similar tab which was designed to close down to a set F-stop for a preview.
And every F-stop no. has an EV number underneath, otherwise it is a well made lens,
This lens is the grand-father of all those Rokkor-PF 55mmF1.7s including MD 55/1.7.