Ok, lets end it where it is. We can correct it if there are mot up to date or reliable information
I know but the number of possibilities is probably limited to E-123, E123 and E 123. If everything is in the right relation, than it is not problem in my eyes…But I can fix it if there is standard for Serbia agreed here.
It can also be tagged on the road with int_ref= but you are right, ref= should be the local number.