J-P: On modern shells, SAGA is from Cartuchos Saga S.A., with factory at Partida de la Caparrella, s/n., 25191 Lleida, Spain. The “SAGA” on the side of the shels is usually in a trademark form - that is, unusual formation of the four letters. Their headstamp is usually SAGA, in normal letters ss typed here, along with the caliber, repeated twice to form a four place headstamp.
The company was established in 1951, and their product is widely exported.
I am sure that “SAGA” is a name, and not a combination of initials, because when it is printed in normal text, using upper and lower case letters, it is printed as “Cartuchos Saga S.A.” and not as “Cartuchos SAGA S.A.”
That is all I know about them.
Edited for removal of typos only.