This is an example of the ultra rare Mitchell Variable Diffusion Filter.
This unit utilizes two gradated Mitchell diffusion filters to increase the strength of the effect as they are slid against each other via a geared track.
Unfortunately the opening is only 3.5 inches in diameter and therefore not large enough to cover most modern lenses. These units were made in the 1920's or 30's and used with early Cooke, Baltar, Astra etc lenses.
Sometime in the late 90's I found a partial frame of one of these devices in a junk box and didn't know what it was until I read John Alton's 'Painting with Light'. Ever since then I have been on the lookout for one. About three years ago I came across this one and immediately grabbed. These diffusers are exceedingly rare and aside from the fragment of one I came across years ago I have never seen another one.