The purpose of the “reserved” pin has been speculated for decades. Remember that Canon started production of the FD lens line in late 1970 meaning the lenses were probably designed and engineered in the late 1960’s. The state-of-the-art of camera design was primitive back then (by today’s standards) so whatever Canon may have had planned was probably pretty simple (like the focal length indicator you suggest).

Here it is, 40+ years since the FD lens line was introduced, and we’re still wondering what use Canon may have had in mind for the reserved pin.

Jim B.