The visors in your first photo don't appear to have the post sticking out that would clip into the center retainer.
https://www.firebirdcentral.com/1970..._p/int-504.htm
https://www.firebirdcentral.com/1970...p/int-1011.htm
I agree, it looks like you had a roof mounted mirror. Maybe the previous owner was tired of the glass mounted one falling off as they often did in the sun.
Does the hole pattern look like it matches this bracket?
https://www.firebirdcentral.com/1967..._p/int-424.htm