Why is there a plate on the rear of some EDLT tubes?

Question

Why do some EDLT tubes have a black plate behind the rear lens and others don't?  

Answer

19 and 50 degree EDLT lens tubes had this plate added to limit the amount of light hitting the edge of the lens.  This allows for a cleaner edge when sharp focused.  

The part number for this plate is 7060A3149.  It is not used in standard definition lens tubes.