Also - found a thread that said: "...One other thing I know of is some ammo with an "SF" head stamp. This stuff is said to have been made for the 1909..."
Can you post a photo from the side showing the shape of the base with a measurement in mm? I think what you have is a French made 13.2mm shell for an M1929 or M1930 Hotchkiss heavy machine gun. The guns were used by several different countries including Greece. I don't know if the guns were all imported from France or if some were manufactured locally under license but I would imagine that when Greece made the initial purchase of guns they also bought French made ammunition with them.
Here is a WW2 era photo of Greek soldiers with the gun used in an anti aircraft role