howdy folks.
I took a few photos of the mirror while it is off.
Here they are :
rear view mirror — not original I am curious what it is off of now.
Maybe a later model Chevy truck or car?
Might be a generic parts store replacement, dunno.
I did see what looked like a date mark inside the day/night level that looked like 1977 so maybe it's from a old square body?
Anyway, thought I'd post a link to the pics to see what can be found out.

Happy Wrenching,
-W