As others have already said, when in installing Stelvio hand guards on the Norge, extra washers are needed, especially on the throttle side grip end attachment to fill the gap between the bar end weight properly. If you don't add extra washer(s) then the throttle can bind.
If you put a lock washer instead of a flat washer, it works just as good and also helps keep things tight. A 3/8" stainless steel washer works perfectly. Of course, use the thread locker as stated in the instructions.
Why Moto Guzzi didnt leave out the spacers for the Stelvio, add a couple washers and call it a Norge handguard seems shortsighted. Wouldn't cost them anything to do but would attract accessory buyers.