The differences in the fence rails are in the configuration of the extrusion where it meets and mounts to the saw table. The old style has a solid channel with unthreaded bolt holes- making it a real pain to mount, while the newer style has a T- track like slot on the back so it is much easier to mount and position. Both have the same top and large fence casting lock T Slot. I have an old, used 52" rail I would sell.
How much do you want and where are you located?