The shutter curtains have rollers onto which they wind, like a window blind. On the II, these have pin bearings -- a simple metal pin going into a simple hole in the shutter crate. On the IIIf, the right side will have a pin bearing but the left will be a roller bearing. The WWII IIIcK cameras had roller bearings on both sides, but that turned out to be overkill. I am not certain just what the arrangement was on the early IIIc production from 1940. I might have to run down Jim Lager and ask him.
I do know that the shutter on my IIIc and IIIf RDST and on my IIIg are all of a sort, and I have no problem shooting with any of them. The IIIc was my first really SERIOUS camera (purchased in 1989, when I was 39), and it is the one they will be prying out of my cold, dead hands after I have left this mortal coil. It is a most wonderful camera, and John Van Stelten has worked to keep it up to snuff despite my occasional, er, bounce testings of it on hard concrete. I carry a Rollei 35 (no RF!) as my glovebox camera, and I have three kits to take out to do shoots -- one with my Leica Wetzlar M6, another with my Rolleiflex 2.8GX, and a third with my Hasselblad 2000FCM. But, when I just want to make some fun photographs, I take the IIIc or a Contax II or my Werra III. Stay focused, guys!
Marc