Reader Harm Metsgers wondered if there was something wrong with (the spark plugs of) his classic, which by the way also worked fine with these 'blown' spark plugs.
We are dealing here with what the English speakers call 'Corona staining'. That is indeed a leakage of compression through the inside of the spark plug. According to manufacturers such as NGK, it doesn't hurt, but it doesn't happen often that all the spark plugs are 'bothered' by it.
There are people who say that Corona staining occurs after tightening spark plugs. But that has not yet been proven
I still want to respond to this. This is not a corona staining as described, this is just a case of a leaky spark plug that causes pressure to leak through. Corona staining is simply the electro-static attracting of (oil) particles in the air around the spark plug. They then burn on the hottest visible part of the spark plug, so close to the metal. But corona staining does not have those lines up that way, so this is certainly not corona staining.