a variation of the common triple-flanged bayonet attachment. The advantage of the breech-lock over the bayonet is that neither the contact surfaces between the body and lens, nor the signalling mechanisms, rotate against each other when the lens is mounted. This prevents any mechanical wear, which could conceivably reduce the very precise lens-to-film distance or introduce communication errors between lens and body. Canon's first iteration of the FD breech-lock, extended forward from the earlier R- and FL-series lenses, used a rotating mounting ring at the rear of the lens; the lens body itself did not rotate for mounting. Its minor disadvantage was a somewhat slower lens change than a bayonet. There are three different versions of breech ring FD lenses: The first version had a chrome plated front barrel (nicknamed "chrome nose") and a green "o" for auto exposure. The second version had a black front barrel and still had a green "o" for auto exposure. The third version had a black front barrel, and a green "A" for auto exposure. These different versions are detailed below under variants. Second-generation FD lenses, first marketed in 1979 as New FD, are mounted like bayonet-mount lenses in that the photographer twists the entire lens body to mount and dismount, though the actual mating surfaces still remain fixed. This retained the advantages of the breech-lock mount while enabling the convenience of a bayonet. The letters SC or SSC, to indicate the lens coating, were no longer put on the lenses. Canon documents stated that All new FD lenses except for the new FD 50mm F1.8 had SSC coatings. Canon later chose a bayonet-style mount for its EOS system's EF lenses, where there is no precision mechanical coupling. Like its FL predecessor, the FD mount system allowed automatic diaphragm function, but in addition, a new signal pin supported full-aperture metering. A second signal pin for the "auto" setting of the aperture dial, plus a linkage to allow the camera to set the degree of diaphragm opening, enabled integral auto-exposure. The first camera to use this was the 1971 Canon F-1, when equipped with the Servo EE Finder. Later, the Canon EF of 1973 had automatic exposure built-in, as did the very popular Canon A-series cameras (save the AT-1) beginning in 1976. Thus, starting with the first FD lenses produced in late 1970, all FD lenses had the capability of supporting full-aperture metering and multiple