Item is a Trifocal Viewfinder. The earliest Bolex model H motion picture cameras included this viewfinder which could be fitted at the top or on the side of the camera. The field of view is shown for lenses of 3 focal lengths. Field of view is changed by raising or lowering side levers which move magnifying prisms into place inside the viewer. When fitted to the side of the film door, the finder offers parallax correction by adjusting a dial which corresponds to the distance between the subject and lens. The H-16 version shows the angle of view for 15mm, 25mm and 75mm lenses; the H-8 version adjusts for 6.5, 12.5 and 35mm. A serial number is located on the rear of the viewfinder which, in most cases, matches the serial number of the camera to which it is attached.