I agree with Musky's thought ref the earth brush. It's the most likely area for reasons he suggests.
To check, a meter from mag spindle to mag body should show very low resistance, close to zero ideally, if the earth is doing its job. (Have to allow for a degree of error at low readings on many meters though.) If you see an obviously high reading there, t'ain't right and could be the problem.
Let's assume, though, that there is perfect continuity between it and the armature inside . . .
Have you tried holding a bare wire (pre-attached to an earth) against the (clean) head of the cb centre screw while the engine is running? Ie bypassing the cut-out arrangement and button completely?
That ought to stop it. If it does, fault is in the cut-out arrangement - however unlikely as you've revisited it several times.
Some basic info if you want to confirm health at the cb end . . .
A meter from centre screw to mag body, points open - about 0.5 ohm.
With the points shut - 0 ohm.
(0.5-0.6 ohm is the approx resistance of the low tension winding.)
From the cut-out wire itself to any earth you'll probably see a few ohms - that's the brush and spring. But it shouldn't be a high number.
If readings are all good and the engine still won't stop using a direct wire off the centre screw, then we're in very weird territory.
