A circlip or snap ring is a type of fastener consisting of a semi-flexible metal ring with open ends which can be snapped into place, into a machined groove on a dowel pin or other part to permit rotation but to prevent lateral movement.

This one is used for fastening the flywheel in place on many vintage Vespas such as VNB5-6, VNC1, VBB2T, 146001 >,VBC1, VLA1, VMA, VLB1, V5A1T 92887 >, V9A1T 25001 >

Vespa OEM Part Numbers:  S05981, S.05981, 674