Fiberglas number boards started showing up in the mid-1970's N&W was an early user, and they glued thin black plastic numbers onto the white fiberglas with some kind of iron glue, to eliminate ever repainting the boards.
Diesel number boards were glass until phased out around 1985. The exterior glass was clear. The painted glass was frosted, mounted behind the exterior sheet.
Yours is a contemporary type, and came from a diesel locomotive. Most are painted black, with white numerals, like yours, but some are white with black numbers.