I see them all the time at cruises around here on early 50's chevys with the visors, or any car for that matter with the visor.
They are generally plastic, and stair stepped, and looks like a D laying on it's back and they mount to the dash. It work like a prism. It refracts the light so you can see what color the light is, because you can't actually see the stop light because of the visor.
Rocky