I, too, think it was a backlit translucent insert for the puzzles as well. The solution area had the lighter background, and there seemed to be a brighter center and darker margins. However, the solution text didn't fade at the margins, which would be the case if it was projected.
As for replacing it, the solution area didn't really pop, in my opinion. Nowadays, the duratrans technology is much more advanced, but it just has a dull look in 1979. You had black text on an uneven background. Not only was an art card cheaper, but it provided more consistency in the presentation.