A little detective story I've just finished illustrates that..
I've been writing up a service guide for the Arri IIC but one thing was bothering me. Every online source of information and nearly every written one I've come across about the IIC - including an American Cinematographers Manual, Carlson's Handbook, Raimondo-Souto's books, even the Arri IIC manual itself - mentions that the shutter angle is 180 degrees.
But every IIC I've looked at or worked on has had a 165 degree shutter, except for one that was 172.8 degrees. And looking at the timing of that 172.8 camera, I noticed that the mirror wasn't covering the gate for the entire pulldown cycle, a corner was being exposed as the film was still moving. A 180 degree shutter would almost certainly create travel ghost. The only source that agreed with my experiential finding of 165 degree shutters on IICs was Samuelson's "Motion Picture Camera Data" book. What was going on?
After some digging around, I finally uncovered this explanation (revealed in the new Arriflex 35 book by Norris Pope, as told to him by Axel Broda):
A 180 degree shutter was introduced with the IIA in 1954, along with the new cardiod cam movement. But problems with the new design were encountered, and so when the IIB was introduced in 1958, the shutter was "quietly" reduced to 165 degrees, and remained so for the IIC. "The exposure difference was negligible, so the Arriflex company saw no need to change the shutter description". A remarkable decision that has caused confusion ever since. The real reason of course was that Arri didn't want to admit their mistake.
172.8 degree shutters came later, to deal with flicker in 50Hz countries at 24fps, but I strongly suspect under certain lighting conditions they will show signs of travel ghost in one corner. I don't know if they were supplied by Arri or were after-market modifications.