I don't know your laws, so won't be any help in figuring out why someone in the past de-rated your truck. Here in the US many States allow the owners to lower their GVW classification/rating to reduce their annual registration fee's. If they get caught going over their reduced GVW rating they get a heavy fine.
I would think showing your data plate to the authorities would allow you to go back to the original GVW rating (8600 KG)...
Mike B
