I got a Schlage cylinder to play with, and I found that instead of being held in place by a C-clip, the plug was held in place with one of those knurled brass caps that is stopped from turning by a spring loaded locking pin.
Previously I have used something like a tiny screwdriver to hold down the pin while I unscrew the cap, but this is fiddly and almost needs three hands. Instead, I found that the plasic tube from a pen is just the right size to slip over the locking bar and hold down the pin while allowing the cap to turn freely. It also has the advantage for reassembly of holding the locking bar and washer firmly in place while you screw the cap back on.
Just thought I would mention it in case anyone else hasn't already thought of it.