From what I remember, you have to have where the plate was made somewhere on the plate (usually in the middle at the bottom) so that if there were any reason for the police to trace the plate, they could see the details of where the plate was made up, and therefore get any details of the person who purchased the plate such as name, address etc...Complicated I know, but if there is no way of tracing it back, I'm pretty sure it's illegal.
Someone will confirm this I hope...