These are also known as doublet lenses and are 5 and 1/2 inches in diameter. They are the internal lenses used in color light signals and color position light signals. All of the ones I've come across are either embossed "KOPP" or "Corning" and have a date of 1935 on them. Common colors include red, green, and yellow. Less common are clear and lunar white. The bluish lens you see in this picture is actually for a green aspect. When the bulb shines through it appears green. I think these are really cool go-withs!