The two numbers/one, two or three letters IS the registration -often carried underwing only. The numbers are the 'departement', the same as used by car number plates.
Some ULMs carry F-J... either externally or on the instrument panel, but this is a radio callsign allocated to the pilot, not the aircraft.
Air-Britain's annual European Registers Handbook includes quite a lot of such aircraft, but details appear difficult to come by so inevitably is not complete.
HTH, Rhys.