Hi
You can not use M6 thread directly to the 3/8" thread on the GenIII.
I did like this...
You have to DIY a bolt that has a outside diameter in 3/8" thread and drill a M6 metric centrum hole thread inside this bolt. Then you screw this bolt to the GenIII four holes and then you can screw a M6 soundcare spikes.
Or you can see it like this....
If you look at the original spike that comes with this speakers and you drill a hole in centrum of the spike, then you would have a bolt that has a thread that is 3/8" outside and M6 metric thread inside.
I hope you understand what I am talking about

Thanks