Kopp Glass lenses 2 in a 6 3/4 3 3/4Kopp Glass lenses 2 in a 6 3 4 3 3 4 Company Name: The words "Kopp Glass" are frequently embossed along the outer edge. Part Numbers: Look for codes such as TL (Traffic Lens) followed by a 4 digit number, like TL 4666. Optical Specs: Markings often include the diameter and focal distance, such as 6 3 4 D followed by a focus measurement (e. g., 3. 125 F). A. A. R. Testing: Some vintage railroad specific lenses will be marked "A. A. R. Tested,"