Rubber gas cap gasket for most large frame model Vespa scooters with a folding-type gas cap. P/PX-series scooters and Genuine Stella scooters do not take a gasket. O.D. 71 mm, I.D. 55 mm

While you're at it, you may need these as well:

Sku: 4435 (Gas Cap Nut Pin Rod)

Sku: S 03212 (Split Pin, Gas Cap Knob, Most)

Sku: 92217 (Fuel Cap Knob, All Vintage Vespas)