Could you give some more detail about the Big Bolt? there was a very large GMC Diesel that had a different cab and a different radio. Did the inside of the cab look like yours?
What is the name on the radio? There were Canadian radios called "GM Auto Radio" that were made for use in the truck and in some cars. The front controls were adjustable in the width of separation and also the front angle. The radios used a different dial glass in different applications as one was upside down from the other.
