I have a Rodenstock Ysarex 127mm f4.7 in a Prontor SVS shutter. I want to remove the lens rear and front cells (glasses) from the shutter. I was able to unscrew the rear cell, but I have no clue how to unscrew the front cell. No threads visible, no notches, no screws... I tried unscrewing it using pressure with my fingers but it doesn't move.