I agree with Pat those are mechanical related and not arcing. Distributer rubbing the fire wall is a good thought, any external marks on the outside of the cap you could have overlooked? Your sure the cap was on correctly, you said the unit itself is in good shape. I to would also like to see the rotor that would be a key shot to put up as Dave Severson pointed out. The button to me shows mechanical or rubbing damage looks like the collar it lives in is chipped . How about those contacts under the scratched area, anything going on there, can not tell from the angle of the photo what the condition of them could be.