-) "FD" refers just to the bayonet in combination with the arrangement of all the actuators
-) "FD-new" is used to distinguish between the older FD breechlock bayonet from the newer "true" bayonet-handling coupling. As indicated this is in first instance a handling/design issue. In same cases the optics are slightly different too.
-) "L" stands for very advanced (and expensive) optical designs within Canon's different lens ranges
When you say that, do you mean the difference between the lenses that attach and then sort of have a screw on lock, to the ones that have a click button lock? Both styles fit fine on my AE-1