You say he was a mural printer? Did he use a mirror under the enlarger to throw the image onto mural? If that's the case, then polarizer would make a lot of sense to eliminate the double image from the mirror. You see, unless you use the first surface mirror under the enlarger, you'll get two reflections - one from surface of the glass and another from the silver surface. It's been a while since I've taken the optics course, but polarizer might eliminate one and not the other as such improving sharpness of the image. However, I'm really just speculating here
I like that explanation. The polariser will have no effect on metallic reflections, but will help to cut down front-surface reflections off non-metallic (glass) surfaces.
Was there a mirror involved when he was printing these murals?