Dear Nick,
Phil did a lovely job fixing that Contaflex II a few years ago. You should find his images very helpful.
To reach the shutter you must first take out both of the two screws that secure the distance scale. You are then able to rotate the distance scale ring around the back of the serrated focus grip independently of the grip (through 360° or more if desired). You will then need to insert a screwdriver through one of the now empty threads in the scale ring you have just removed those screws from, in order to access, and loosen the three set screws beneath the scale ring, which are what lock the serrated focusing grip to the lens mount of the front lens piece. You'll need a good light, a sharp 1mm screwdriver and a gentle touch to find those small slots in the recessed screw heads. You must rotate the distance scale ring, until its vacant thread holes align with the heads of the focus ring retaining screws underneath the distance scale ring, to reach those three screws that secure the serrated focus ring.
But first, it's critical you take a sharpie or a dab of white liquid paper and mark the position of the serrated grip relative to the mount (Ie the ring which has the lens serial number along with "Carl Zeiss etc" on it) when focused at the infinity stop. Your infinity stop for the lens is part of the that serrated grip, you see. If your infinity setting is going to work out first time you reassemble, you *must* have the both the lens mount and the grip positioned in the same locations they came off. You should know if you're good to go, because during disassembly, you will clearly see three small dimples in the outside circumference of the lens mount after you have loosened its retaining screws and slid it off the mount. If at reassembly, you have positioned it well, the tips of the grip set screws should find those dimples easily, when you proceed to tighten them.
So that's the first critical point. Set infinity and align grip and mount to each other. Take a digital pic from the front showing the relationship for good measure too.
Having done this, you may—keeping the grip at infinity—loosen those 3 recessed screws for the grip (no need to completely extract them) and lift off the grip.
The multi start threads for focusing this front cell focus 45mm Tessar are cut directly into the outer circumference of the front lens cell mounting and the inner circumference of the middle lens cell mounting.
You already know the orientation of the front mount at infinity. To open up the shutter the middle mount needs to come out but first the front cell must be unscrewed from the middle cell.
Because the threads are multi start—helical—starting from its infinity position, you must note how many degrees the front rotates before the threads reach their starts. Note the orientation of the front cell and its mount at the exact moment the threads separate. You will obviously need to use the same thread start on re-assembly, if your previously marked relationship between front cell and grip is to come out OK the first time, when you re-install the serrated grip and lock in the infinity focusing distance.
The abbreviated version: mark grip and front mount relative to each other, at infinity; and mark the position the helicals part when the front cell comes out.
If a Contaflex I or II fails to run correctly, apart from any issues with the shutter proper, it's likely that the actuating rings behind the shutter require cleaning and lubrication. This means taking the shutter right out, necessary anyway for correct servicing, which is easily done by removing the retaining ring through the inside of the film gate. Note that the pre-tensioning of the aperture stop down will be lost when the shutter leaves the body. This is the purpose of the small toothed wheel inside the film gate. After the shutter is sorted and correctly re-fitted, you must remove the lock screw, loosen the centre screw, and re- tension the spring before the aperture will stop down before exposure.
Due to the location it is impossible to get a driver blade onto the screws for the wheel. You'll have to angle the tips of a couple of drivers (either by chamfering their edge to an angled blade, or heating and making a gentle bend in the shank at an acute angle not too far from the blade edge). This will enable you to loosen and fasten the screws without risking damage to their slots which might then dictate a total strip down.
The learn camera repair site has a copy of the Contaflex I–IV service manual published by ZI available to download.
Interestingly there were a few changes to the lens front from commencement of production. The rate of focus change from infinity to minimum (total degrees of front cell rotation) was altered at least once, maybe twice (each version needing its own grip with the appropriate stops). A shift from male filter threads to the more common female threads also occurred. The very earliest examples with an older speed scale and 1/500 booster spring used the old version of the Synchro-Compur shutter based on the Compur Rapid. It has no self timer, ten aperture blades instead of the later five, and used a different shutter mounting plate to later build examples which had the new speed scale shutter with self timer. You could say it's a smaller sibling of the one fitted to Eg a 2.8C Rolleiflex but modified for SLR use.
The manual is quite explicit about not being able to graft later shutters into a very early version, unless the whole front mounting for the later shutter is also fitted with it. For these reasons, obtaining the factory manual is highly recommended, because the shutter actuation of the oldest build examples is rather different from those which emerged during 1954.
Cheers,
Brett