almost useless truck trivia

[Axle]

Noticed something I've not noticed in the past while swapping the mirror from my old '83 into my new '81 GMC.

The rearview mirror that was in my 81 GMC has TRUCK etched into the glass. My '83 Mirror has no such markings. The '81 mirror also has a rough textured day/night lever. Everything else about them is identical. Construction, Coloring (and discoloring* even), shape, size, viewing area, Made In Canada embosing on the bottom.

So I guess that begs the question, what makes the truck mirror unique that it needs to be identified from other mirrors?

Alex.
*the silvering has not degraded on the '83 mirror.