R Adaptor with variable ND Filter

Sorry if I've missed it in the deluge of info, but has anyone seen further details about the EOS R to EF adaptor with the variable ND filter. I can see it for pre-order on a few sites but details are sketchy. For instance, what range of stops does the variable ND offer? Will third parties be able to make filters for it?
I shoot with a 5DSR at the moment and I'm no rush to move to mirrorless but I think this is a genius idea. ND filters that could be used with the 11-24 without having to carry a massive adaptor and dinner plate sized filters would be superb.
 

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