Jaegers catalog #18B 1676. This cooke triplet is inside of standard projection mount and was intended to be used in a slide or film projector to display 35mm or 2-1/4" slides. Lenses were manufactured by the American Optical Company, and can be removed by unscrewing the top of the housing. The front part of the mount is knurled and the shank is spiral grooved. Outside diameter is 2-1/2", length is 4-1/4" and will slide into a tube having an inside diameter of 2-1/16". Focal length is 5" (125mm), clear lens diameter is 36mm, optics are fully coated. Unused.