"Found On The Dashboard Of An Old Gmc. A Lucite Like Material, With Ridges"

Answer: "It's a traffic light viewer. Traffic lights back in the day were mounted on the same side you stopped on which sometimes made it difficult to see the light if you were first in line, this helped reflect the light from above so you could tell when the light changed."
Pretty sure you are wrong.... the black/blue pages in those are thin. photo looks more like an album.
Better... er... what? That would depend on the specific function of the slide rule. I used these in an engineering drawing office early 70's
#56... That is what is left after removing a metal screen that was painted at one time
came with firearm accesories... Probably a wick for old gun.
But the two things can look more or less identical.