Think I may have found a culprit..

I decided to pull all the plugs to look at them (and replace them).. 7 of them looked like the images I posted previously.. then there was this one..

Am I correct in surmising that this was not firing? It was in the cylinder at the back of the engine on the drivers' side.. the hardest spark plug boot to get to, since the power brake booster is right in the way, and the plug boot rests against the header (I have fiberglass heat insulation boot on the plug boot)..