S9 registration
The S9 part of the registration is Sao Tome & Principe, a small group of islands of the west coast of Africa. Once owned by the Portugese. I think it must be a place of convenience to register old aircraft.
As an aside, Registration marks on aircraft generally follow from a historical sense the call signs alloted to each country in the 1930s by the ITU (Internation Telegraph Union). Hence Axx and Nxx was given to the USA. But only the N letter used for aircraft.
So sounds like a strange load or task?