But a licence plate is a “number or other mark placed on a vehicle for the purpose of distinguishing it from other similar motor vehicles”. You may say it is not, but whether it's numbers or letters, it is a mark--at least it is a mark--and those numbers and marks are placed on a motor vehicle precisely for the purpose of distinguishing it from other motor vehicles, because every licence is different.
The officials seem to be very assured about this. I am not. But I'm willing to stand down here and let officials respond. Colleagues may want to ask more on it.