This is the "Breech Lock" version of the Canon FD 50mm f/1.8, identifiable by the silver mounting ring at the base. The "S.C." designation stands for Spectra Coating, which was Canon's proprietary multi-layer coating designed to minimize flare and improve color balance. This lens was the standard "kit" lens for iconic bodies like the Canon FTb and the early AE-1. It is highly regarded for its sharpness, lightweight build, and the classic rendering typical of 1970s Canon glass. The breech-lock mechanism ensures a secure fit without the friction on the camera's mount that later bayonet styles introduced.