Happ does call it a 7mm clip for some reason, but that is a pretty strange way to describe it. Retaining rings, e-clip style or otherwise, are typically classified by the shaft diameter they are designed for.
Since the Comp & Super have 10mm shafts, you would expect that they need 10mm e-clips. However, I just measured a Happ "7mm" e-clip with my calipers, and it conforms exactly to an 8mm e-clip as shown on this chart:
http://www.oscarcontrols.com/tmp/eclip-metric.pdf. It appears that IL overcuts the groove for the 10mm shaft as if it were an 8mm shaft, and the groove diameter for an 8mm clip is 7mm. That is most likely where the Happ "7mm" clip identification comes from.
All that said, your best bet would be to take the shaft with you to the hardware store....