Richard,
The electrician I use is not available for work now, but he found faults in these CAV control boxes. I have had a number rebuilt by him and if I recall he removed the old innards and replace with the workings of one of the common 60's period boxes, might be an RB108, would have to check at work. He always tested these with the dynamo on a test rig, to set them up.
If you do clean the contacts, wipe some paper through afterwards else you may have a bit of muck holding the points apart.