Lambretta steering lock aluminum covering disc This is a Welsh plug to cover the steering lock mechanism in the headset bottom.
INNOCENTI 15088003See fig. 38 in the image below.
The Complete Spanner's Manual: Lambretta Scooters by Martin 'Sticky' Round Third Edition 8100072
The Complete Spanner's Lambretta Kit Book
Eibar Lambrettas 1954-1989 by Jaime de Prat Salomone
The Unofficial Lambretta Manual Update by Mark Haines