Just to collaborate. I was recently working on a drive sprocket for my 500 Falcone. The sprocket's securing nut is locked in place by a small, headless screw. Just 5 or 6 threads with a slotted top for a screwdriver. Surely what I would call a setscrew. The parts manual refers to it as "Grano di fermo" or locking screw.
Patrick Hayes
Fremont CA