I don’t know this for a fact but I believe the “J” will mean made in Japan under licence at a local factory near The Kawasaki main plant . I did hear the OE tyres are generally harder than say other tyres or even the same tyres aftermarket. This is because the Bikes and tyres will be stood around inn Japan then on a boat then at distribution warehouse , before sitting on a shop floor and could be potentially there for years . This change in rubber helps prevent them from losing their shape and stops the rubber going off .
I’d love to hear the views of any tyre specialist or suppliers we may have on here .