The Fox plug worked great. One test flight to confirm the engine was adequate, then pattern after pattern. I've had excellent luck with the Fox brand. Most of my stock of those is old, as Dan notes. I have some newer Fox plugs (orange package vs red/white/blue,) but the older ones are my go-to as known good.
The K&B looked fine; shiny wire. Though it makes sense that it had some barrier. If I was really well organized, I might look at it with a microscope. I was even wondering if somehow the catalyst chemical, iridium in the platinum family, had somehow departed.
thanks for the discussion and information,
Peter