Alain,
This may be off topic to the question, and I may be very wrong on this, but I suspect the black plastic is a replacement. I base this on the fact that I recently replaced my 'pitted' chrome mirror with a plastic one (since apparently chrome was determined to be a safety hazard).
Now, on topic, I know I've seen some posts here that discuss replacing the glass alone without removing the head...is this a solution to you?