This over sized stepped exhaust port stud is to repair ROUND type exhaust port cylinders where the threads have pulled out. Tap the cylinder 10 x 1.5mm.
An MB Scooters design.
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