It depends on which ones you get. They all mount with bolts into the back of them. It looks like some have one bolt and others have two.
http://cgi.ebay.com/ebaymotors/Musta...55783398QQrdZ1
http://cgi.ebay.com/MG-1600-Lotus-AC...QQcmdZViewItem
http://cgi.ebay.com/ebaymotors/Class...61439765QQrdZ1
They are usually found under Cobra or Hot Rod mirrors.
I hope this helps. I have a set in the attic if you want more info or measurements.
Mike