Hi,
Although nearly all of the original 1930's CK859 that we see today have a 26.5 SOB inside, there were also some with cal 30 and with cal 23.4SC
Here are extracts from a 1940 French catalog :
Note that in this catalog, the one with caliber 30mm is 0.5mm wider than the 23.4SC....
I've never seen a CK859 with a cal 30mm in the metal, and the only one with a 23.4SC I could get is complete and original but a real wreck (don't pay attention to the dark circle on the dial, it is the reflection of my camera lens...) :
This leads me to think that both 30 and 23.4SC were manufactured in a much lower production and are significantly rarer than the 26.5.