Re:
The problem is that there were / are a lot of makers of diver's helmet gear cable stops and consequently, no real "standard" for internal diameter. They are normally around 4 mm / 3/16"
Additionally it might have been drilled out, or otherwise butchered at some point, either pre- or post-frame-build, for some reason. Some stops are a simple cup (but not judging by the images you provide, in your case ...)
You'd definitely be well advised to fit a ferrule for the simple reason that longitudinal compression on gear outer tends to cause the strands to migrate out of the ends of the sheath - and this will eventually happen in the situation you have pictured as well. This will definitely give you a problem with the indexing as well as being unsightly.
If you can measure the ID of the cable stop drilling (even by inserting the tail of various-sized drills, for instance, as go - no-go guages) we might be able to help with a ferrule ... various different ones here ...