Which kickstart return spring?

I need a new return spring for my very late (1965) eibar series 2.
Presumably I need one for a series 2, but as it's so late maybe I need series 3?
Without knowing the difference between them I've no idea how to decide.
Anyone know?
Presumably I need one for a series 2, but as it's so late maybe I need series 3?
Without knowing the difference between them I've no idea how to decide.
Anyone know?