If you have the gun laying on its side, barrel pointing to your right, back of the gun pointing towards your left, then the left hand one is dwell down, the right hand one is dwell up.
Its been about a year since I've fired my Impy cause its not currently in working order, so I can't remember the sound of it.
However if you bump into a spare front reg mount, or someone selling them, please point me in that direction cause I would love to get it going again