This is a rare piece of clear filter for some old lenses which do not have the filter thread, such as pre-war Zeiss Sonnar 50 and 135. The original optical part has serious cement problem and is now replaced by a Tiffen Clear filter. The other part is heavily brass and which will fit the look of your vintage lens. Example shown here is on a black nickel 50/2 pre-war Sonnar with the very rare bakelite rear cap.
This filter also comes with the original B+W leather case in very good used condition.