OEM Piaggio Outer Pulley, also called Outer Variator Plate or Outer Finned Variator Pulley for Vespa, Piaggio and Aprilia 250 and 300cc Scooters. This gets worn over time by the belt, and can be chewed up if the nut isn't put on right.

APRILIA Atlantic 250 

PIAGGIO MP3 250
VESPA GT 60° i.e. 250
VESPA GTS i.e
VESPA GTS i.e. 300 
VESPA GTS Super i.e. 300 
VESPA GTS Super Sport i.e. 300
VESPA GTV i.e. 250 
VESPA GTV i.e. 300