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4.7 mm...
Well if they aren't already threaded, cut 6mm threads into the holes in the GSO rings. For a 6mm thread it is normal to start with a 5mm hole, 4.7 should be close enough that a 6mm tap will start and cut into soft alloy.
For this you will need a 6mm metric tap and the handle to grip it, not overly expensive from Bunnings. Don't buy an entire set of taps and dies if you don't need to.
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