Check the shutter curtain for pin holes after separating the stuck part. See .
I repaired a Series C just over a year ago. I did not have to do more than clean the lens after removing the front and rear lens groups from the main barrel as you have done. The lens barrel is magnesium and frequently cracks or fuses itself to the other parts so having it separate from the main barrel is a good thing. With the front lens group removed from the main barrel look at each end to see if it is internally threaded. The only picture I have of the lens is the front and it does not appear to be threaded and there are no spanner notches either. The front inner ring with the lens info has the gap between M.M. and f2.5 centered at 12o'clock and the word Lens centered at the 6'oclock positions. A large rubber strip that fits the ring only that you can put pressure on can be used to see if the ring will turn. If there are spanner notches or a cap ring on the rear of the front group then at least the rear element is removable by removing it. Also look closely at the outside of the barrel for very small set screws at the retainer ring positions as that was a mounting technique in that era. When removing a ring secured by an external set screw count the number of turns to remove it and reinstall the same number of turns to maintain correct lens cell spacing.

Post some pictures of the lens sections.