some telephoto lenses have black barrels, but their date code, lack of an aperture lock button and freely rotating breech ring place them into the first version FD lens group. The second series of breech-lock FD lenses (1973-1976), which discontinued the chrome barrel front, is engraved "S.C." in white or "S.S.C." in red on the front of the lens. The S.S.C. coating was extended to most lenses in this series. The basic S.C. coating was, for the most part, limited to the least expensive lenses. These 2nd breech ring generation FD lenses still had a green "o", but now there was a chrome lock button on the aperture ring that had to be depressed to put the lens on or take the lens off of "o". One still could not put the lens on "o" unless the camera body had the AE switch pin hole. And the lens could not be mounted on a non-EE / AE body if the lens was set on "o". The breech ring now featured a lock which prevented it from rotating unless a rear cap was put on or the lens was mounted to a body. Further, the breech ring rotated slightly when the lens was mounted to aid in getting the lens securely mounted. The aperture diaphragm was opened fully when the breech ring was in the locked position. The third variant, made from 1976 to the end of production of breech ring FD lenses (around 1980) changed the green "o" to a green "A" and the chrome aperture ring lock button was changed from chrome to black. The breech ring was still locked unless mounted and it still had the spring loaded twist to make it easier to mount the lens. The third version FD 50mm F1.8 lens also received a plastic front barrel to reduce size and weight. This corresponded with the introduction of the Canon A-series cameras which were smaller and lighter than the older, larger and heavier "F-series" bodies. In 1978, with the introduction of the New FD series (nicknamed 'FDn'), the coating type was no longer specified on the lens front. All of these lenses received S.S.C. coating, with the sole exception of the 50mm lens. The original breech-lock FD lenses and the New FD lenses are completely functionally interchangeable, and may each be used on any FD camera body. A minor operational difference between New FD and