I am not quite sure what has happened here, but ...:
After having been out in the rain and using my Nikon Ai 300mm f4.5 lens, I just tucked it away in my camera bag and forgot it there. It was probably wet and it seems to me as if water has creeped in between the lens tube and the retractable lens hood and caused the rubber fodder of the lens hood to partially dissolve. When moving the lens hood back and forth, crumbles of black, gooey rubber are falling off and sticking to the front glass if I don't have the lens cover on. Under the black coating, the bare metal is appearing, which is optically not an ideal thing.
Is there some obvious way to remove the lens hood from the lens tube, so that I can get the old coating completely removed and replace it with something else? Perhaps a very thin sheet of felt or something similar. There are unfortunately no obvious screws or other tell-tale signs of how it is actually attached.