Split pin which secures the gas cap knob to the fuel tank on all Vespas. Also secures the engine cowl securing assembly on VS1-5 only.
While you're at it, you may need these as well:
Sku: 4435 (Gas Cap Nut Pin Rod)
Sku: 2319 (Gas Cap Gasket, Vespa LF)
Sku: 92217 (Fuel Cap Knob; All Vintage Vespas)