What do you think of these spark plugs?

The end with the spark gap is primary importance . What are you burning ? I have not seen that much crud since the leaded gasoline days. This much fouling would be typical of leaded fuel , not a worst case. At that time the deposits were very complex ; some combination of lead oxy sulfate with bromine. No significant difference between cylinders. You might try platinum plugs , although I never used platinum with leaded gasoline. Another thing that has changed along with lead removal is significant reduction of sulfur. So, I have no clue other than it looks like leaded gas fouling.