Lambretta Layshaft rubber O Ring.
99% of engines that leak oil around the rear hub bearing are due to this O ring not being changed when the engine is rebuilt. It is a false economy not to change it.
INNOCENTI 19030032
MBB0084
Anello di Tenuta Oil seal Anneau d'étanchéité Dichtring "O" ring 20.0mm X 2.0mm