In various editions of the Harry Potter books, films, and other promotional materials, different versions of the flying Ford Anglia's license plates have been shown.
Harry Potter and the Chamber of Secrets, UK edition
HOW 777
The full license plate can be seen on the original drawing, as sold by Sotheby's.
Harry Potter and the Chamber of Secrets, film
7990 TD
Pottermore
COS 207
Harry Potter and the Chamber of Secrets, illustrated edition
HOW 782D
Besides for Pottermore, the rest don't seem obvious. What is the significance in the licence plates?