Peace of mind at last. Have run a 16t Wemoto front sprocket for about a year and was never comfortable with the lack of a rubber cush drive as on the original Honda 15t. A solid drive cannot be as kind on the drive shaft as the rubber bushed job particularly if there is any snatch from the chain. The Honda part no is; 23801-MN8-000 kindly provided by several replies on this site and yes it is a 13 tooth spline. cost £25 from David Silver. Swapped the Honda for the Wemoto 16t and all well after filing some extra clearance on the chain guide which sits in the plastic sprocket cover. I swear the ride now sounds quieter and smoother.