As it is cold outside to-day for final garden pruning, I have done a little experimenting with the copal/sinar shutter. I have come to the conclusion that the internal spring is so solid that there is no need for a coil spring to be built into the release cable mechanism. Even if the actuating rod does stick out of the threaded sleeve a little, the friction on a short and straight release will not be enough to accidentally trip the shutter. So we are talking about a very simple design.
The attachment thread appears to be an ordinary M4 with 0.75mm pitch. Unless Sinar/Copal had their own variant? Bob S would be useful here? But perhaps these cables are still available as new items?