MB Scooters have remade these gear box shims in the UK from hardened shim steel cut to fit perfectly on all Series 1 2 and 3 engines.
The Complete Spanner's Manual: Lambretta Scooters by Martin 'Sticky' Round Third Edition 8100072
The Complete Spanner's Lambretta Kit Book 8100027T
Eibar Lambrettas 1954-1989 by Jaime de Prat Salomone
Lambretta and Vespa Street Racers BOOKAZINE by Stuart Owen