Picture? I think because you have a fire truck you may have a non-GM decal affixed. It is contrary to the accessory nature of visors to see it affixed there on AD's. The vanity/visor mirrors sold by the dealers would make this placement unusual. Why place a decal where a customer may place a mirror?
https://www.flickr.com/photos/advance-design_parts_co/22920144840/in/album-72157631868452754/
Just an example of several styles available.
It is also unusual (except deluxe cabs) to find a RH visor, making visor placement even more unusual since the DS is much more likely to be your only one.
https://www.flickr.com/photos/advance-design_parts_co/23479714204/in/album-72157631868452754/
Maybe one of the Canadian bolters will chime in soon?

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr