I have a 150mm Nikon El-Nikkor enlarging lens that has some dust inside it and what appears to be the start of some fungus. I have taken apart and cleaned and re-assembled dozens of different lenses from 35mm camera lenses to large format lenses to enlarging lenses with no problems. However, this lens is driving me nuts. I cannot unscrew the front ring. Does anyone know which ring needs to be unscrewed? Maybe I am trying to unscrew the outer ring when I should be unscrewing the inner ring that has the writing on it?