Two power magnets for the SL100 series - screws included.
With an extra set of magnets, one for each wheel, you will get even more light out of the SL100, as the light will flash twice as fast. At a normal speed of approximately 20 km/h, the flashing frequency is approximately 200 flashes/minute with one magnet and 400 with two magnets per wheel.
The SL120 flashes at a fixed flashing frequency of 120 flashes/minute, and adding an extra magnet will not change that. However, with an extra set of magnets, the backup function in the SL120 recharges much faster, which is clearly an advantage if you cycle in cities and often have to stop at red lights.