Unable to remove Durant mirror

[Brage]

Hi!
One of the bolts that holds down the Durant side mirror is turning with the nut. I have absolutely no idea how to get the mirror of without breaking something. Do you have any tips?

[WFBowen]

Do you have the large 1968 type or the smaller one used on late 356, 67 back 911/912? The late one has a recessed glass with a black plastic insert, the early one is smaller without the black insert.