That "reserved" pin is found on the very first FD lenses still with chromed filterring up to models from the 80s, maybe even to the very last model. In literature I got there is no explanation other than for possible future use. On the net I only found one source, hinting at an indicator of the focal lenght of the lens, staggered in three steps: short, medium and long. Even if that would be true, it does not make sense to me. What use would that information have had at all? More so as early as 1971? In the latest FD-cameras there were AE-programs for those different focal lengths-groups. Did one have such in mind as early as at the start of the FD-range? Hard to believe.