What George said - don't remove it unless you absolutely HAVE to. On my '75 it was held on by about 4 little nuts (they might have been plastic) which were quite tough to get at. There should have been 6 nuts, but some of mine were broken already from POs.
Once I got it off (I was getting the car painted), I realised it was, as George says, just a general degredation in the reflective coating on the inside. Doesn't look like it can be fixed. Mine will go back on, as is, probably using the same bodgy double sided tape method as the PO used at the end where the plastic studs/nuts are broken
