The iris suggests optical to me, and an old British metric micrometer hints at scientific application rather than engineering. Looks as if it was bolted down in use, not hand-held.
The measuring end (if that's what it is) appears to have at least one part missing. It appears that turning the micrometer screw would wedge open two halves of a sprung holder designed to take a round object.
My guess is that it's part of a larger system used to test optical lens. Newtons Rings perhaps, with the micrometer used to accurately centre the lens, and the iris used to sharpen up the image?
Dave