Leica M Accessoriesfind it on keh.com.
It's an OROLF lens turret with handle. Mounts to a Leica M2 or M3 camera and holds three screw-mount lenses for a faster lens exchange. Has its own baseplate which fits onto the bottom of the camera. Produced for one year in 1960, with production numbers at 250 or less. EX+, $2,029, find it on keh.com.