Here read this thread:
332-428 Ford FE Engine Forum
The spring arm on the rotor in my Mallory distributor wasn't touching the carbon button on the cap (about a .05 gap) and this is how they looked after less than 100 miles:
Here's a picture of an unused cap:
I think the arcing between the carbon button and rotor wears the carbon out very quickly. In the above pictures you can see the pitting/burning of the contact arm on the rotor and the cratering of the carbon button.