Interestingly I have seen the front face of the piston of some KEF B139 drivers - originally black - become a very similar colour after several decades. The front face is an aluminium skin laminated to the expanded polystyrene piston, treated in some way to make it black. From its appearance when new I always assumed it was a black plastic film, however perhaps the aluminium was anodised. Either way it may possibly be an identical surface material to the knob in question. That suggests a possible long term atmospheric effect, happening only in some locations. Unlikely to be oxidation but perhaps due to a contaminant in the air, such as a trace of sulphur dioxide which is a common pollutant.