Cemented achromatic negative doublet lens, box is labeled "not in catalog". 2-1/2" x 2", then tapers down to 1-13/16" x 1-1/2". Edge thickness is 3/4". Focal length is -111mm. These have an amazing blue/purple anti-reflective coating. This was probably used in conjunction with another lens to make a great optical instrument lens assembly! A rare unlisted Jaegers negative achromat!