I believe it should have a minimum radius of 2.5mm which is common to all interior fittings that can be contacted by either a 165mm or 100mm sphere dependant on where it is in the car.
If it is a standard M10 thread can you not just buy a couple of round plastic knobs from Eblag and fit them temporarily for the test? it should still be possible to undo and do up, if not easily and then swap them back after.
The ListerBell ones are M8 and although mine, Ian's and several others have passed; technically and if the tester was having a bad day they could have failed due to the lack of a radius on the centre boss. Not sure if Craig has altered them slightly now.
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