UK registrations change twice per year, in March and September.
Therefore March 2002 would be AB02 CDE and then would change in September to AB52 CDE
Then same again in March 2003 AB03 CDE and September AB53 CDE etc
This system started in September 2001 with "51" plates, before that the first letter of the number plate related to the age of the car, for example X123 ABC is September 2000 to March 2001 then Y123 ABC would be March 2001 to September 2001 etc.
Hope that helps!