it is pretty much every part that is metal inside distributor, except for the reluctor wheel seems to be clean......the weights, part of pickup that bolts to dist., etc.
Apparently this is an issue of certain production years/runs...there was no protective coating/grease applied to metal parts and the caps did not come with a vent (although my cap had a vent) or not enough venting. And due to the MSD producing a much greater spark, it causes an ionization.
I do not know the age of the dist., as it came with car when I bought it......it looks brand new, but I do not know for sure, so newer ones may be o.k.