Hi Piero,
The electrical system is positive ground.
1967-68 Atlas and G15 models had twin Lucas 45110A coils (D=48mm, 12V) according to the Lucas equipment list and some pictures I've seen. It was the standard coil used by BSA/Triumph at the time.
Coils are wired in parallel and each coil is triggered separately (no lost spark).
The wiring diagram is found in the NV 1966-76 Workshop manual.
- Knut