Happy to chime Richard, but have been looking at the pic and not knowing what to think!
First thought was goop used to embed a modern condenser, but I couldn't figure the threads or thin filaments in the muck, and then the condenser in a K2F is the opposite end from the condenser, and said goop is unlikely to swap ends if it self-detaches. There isn't much room to go walkabout..
Then I also thought some sort of silicon sealing goop applied round the plug leads/acorn nuts to keep fuel and water out. It doesn't really look like slipring material from what I can see, but you never know. There are white sliprings, but the form and shape of the goop looks odd to me, and as you say, the slipring would look emaciated through the pick-up holes if that much had been carved off it. Sometimes, if a brush has self-detached the bits can do some carving, and the spring, if it's still there, can make a bit of a dog's dinner of things. Been there and had that with one of mine not so long ago. Still ran (badly), but the slipring plastic was scored to a depth of about 1/8th of an inch and the step between brass and plastic was huge.
Could be some sort of resinous coating that has peeled off the armature windings I suppose, for not being heat-resistant enough, or not able to expand with heat. One does see some coils that look like a resin egg, solid as a rock and shiny/smooth all over, but would they or could they shed bits like this? I don't know.
Unless I could identify it for sure, and found it was definitely harmlesss and not capable of happening again, I think I would open the mag to see whether there was anything bad starting to happen though.
Not much help I fear - but will be fascinated to learn in due course, if an answer comes out in the wash that is.
