The FL lens mount was Canon’s second SLR system, in use from 1964 to 1971. FL mount replaced the older Canon R lens mount. In 1971 the FL lens mounting system was replaced by a very similar FD system. Although there were some differences between the two systems, FD bodies can use FL lenses and vice versa. The main difference is in automatic aperture stop-down. FL lenses (and FD lenses mounted to FL bodies) must be manually stopped-down before firing.
Canon produced some innovative lens designs during this period, like the 19mm f3.5R, which was an ultra- wide lens that allowed for viewfinder focusing by using a retrofocus design. The FL 300mm f5.6 S.S.C. was also groundbreaking by being the first tele lens to incorporate a flourite-coated element to combat chromatic aberration.