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.
![EDLT Lens Tube Back Plate.jpg](https://support.etcconnect.com/@api/deki/files/1655/EDLT_Lens_Tube_Back_Plate.jpg?revision=1&size=bestfit&width=540&height=405)