Guitplayer- That one looks like several I came across this morning that turned out to be early to mid 1960's Truck rear view mirrors.
For Ed, below are some more screen grabs of 1955-59 GMC and Chevy inside rear view mirrors installed that I snagged from the web this morning. Some with the speaker box and some without. I did rummage through my electronic 1955-59 Chevy FAM and found there isn't a picture or diagram of the inside mirror install anywhere in there - blah! I also looked through a 1957 Chevy Accessories Installation manual. I found the diagram and instruction for the speaker box, but nothing on the inside rear view installation. Anyhow, hope these pics will help you figure out your install Ed.
