That can also mean it's lean at time of ignition at the spark plug and that would correspond with clean plugs and electrode wear.
Just because you give a cylinder an AFR of 12.5 to 1 doesn't mean that's the AFR at the plug when you fire it.
Shrinker on electrode wear:
I dont know anything about the engine but the photos appear to show a slight rounding of the corners of the center electrode. At the very least there appears to be a clean zone on the corners. If this is the case then its likely that the mixture is insufficiently vaporized at ignition time. When the atmosphere is low in gas content around the arc location the oxygen burns the metal of the centre electrode. And it usually does this on the edges because thats where the electricity runs. Sometimes this deterioration is reduced by using a more powerful ignition.