Yup, Corps Diplomatique, ay per the Vienna convention on diplomatic privileges where all countries have some form of special numbers for guest diplomats accredited to their host foreign ministries.
Think over in UK / London CD plates just have a small "D" for diplomat, although I think ambassadors cars have the first three letters for their country, and then a 1.
EG Germany would be "GER 1" for Monsier L'ambassadeurs official car, while his private car(s) and your "normal" diplomat from say Norway for a change would have "225 D 150", where 225 stands for guest country, Norway, D for Diplomat, and 150 is the internal number allocated to respective diplomat from the ambassadors private cars on to all other personnel with diplomatic status on a random basis.