Lambretta series 3 pre 1966 (think slot) throttle tube by Scootopia
See fig. 19 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