I have looked at this for quite a while, and I've come to the conclusion that the numbers were painted on, or an ink used that over time, the vehicle in the ink, or paint, etched the aluminum permanently? I'll bet it was nice looking, and bold, when new...
.... How it's held in the light has a bit to do with how it looks as well, it is really kind of neat in hand... But, correct for when and by whom? best to all, til....lat'r.....GT