Sacramento Northern GP7 #711, Circa 1976

Electro-Motive Division (EMD) GP7 #711 of the Sacramento Northern circa 1976.  In every picture I've seen of this engine, that one number board has a white patch in the blank spot next to the 7.  A piece of plastic or cardboard?  Ran out of black blanks?  I'd love to hear the story if anyone knows what it is.

All 11oz and 15oz ceramic mugs are microwavable and dishwasher safe.  While they are very durable, they can be scratched by hard metal edges.  Avoid putting them next to silverware in the dishwasher.