Lambretta clutch center nut to fit all series 1 2 3 and GP models
INNOCENTI 19030007See fig. 22 in the image below.
The Unofficial Lambretta Manual Update by Mark Haines
LAMBRETTA - ILLUSTRATED GUIDE TO THE IDENTIFICATION by Vittorio Tessera
Lambretta Performance Tuning and Conversions - 8099990
Lambretta Tuning Manual by Dave Webster - 8099992