Not a big difference IIRC. I think the 6R has slightly bigger carbs, tighter springs and the front rake angle is different on the 6R for faster steering response. I think the engine/frame/bodywork was the same. Biggest difference was the suspension.
I have not seen any ZX6's. There was the ZX600r that was an entirely different bike (based on the GPZ550 engine with water cooling added.)
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