Trav, I've mixed them up back in the boxes, now, so I don't know which is which. And to be honest, to my eyes, three don't appear any different from the others. I think it is just the shadow and angle of the light.
Because I think I've heard pinging from this engine, I've been watching a lot of gauges over the past few months. LT fuel trim, on both banks, is always near 0. It seems to be right on the money. Every indication I get is this engine is not pinging, or at least not pinging in a way that's harmful to it, and that it's in a very good state of tune.
I used Denso plugs as replacements. The owner's manual gives both NGK and Denso part numbers. Our MDX had Densos, and I used them here, too. I think it's just a hair smoother, and the pinging noise may not be as pervasive at part throttle, anymore. It could be in my head, too!