Very unusual military cemented triplet lens, coated. 31.8mm diameter by 21.2mm center thickness. Top element is dome shaped. Bottom element is tapered to a 12mm clear opening. See photos. Lens has a negative focal length of -83mm in one direction and -28 in the other direction. The interesting thing is that you can look thru the dome and see an enlarged image of something placed under the bottom edge. Military NSN #66500114255540. Made for some military optical instrument. Unused. Only have a few.