Hi Rob,
The Slick 4371 is a 20degree lag L/H rotation magneto, so yes it is an impulse mag. If fitted with two its probably best to hand swing the aircraft with both 'on'. The idea behind grounding the non-impulse is the reduce the chance of the engine kicking back from the non-retarded R/H mag.
The 0320 is usually fitted with the 4373/4370 combo, the 4373 being a 25degree impulse.
Having said that, a double impulse setup would always be better for hand swinging
Kingy
(a bit of a Slick mag geek, having just bought a load at vast expence...!)