The effort to remove and replace a number plate of this size would cost more than they are worth. At least on working steam locomotives. Locomotives on the scrap line would have definately lost any metals that would be used and many of these engines were completely cut up for scrap while others deemed useful for parts lost what they could afford to lose.
The material is is made from closely resembles the bell (a brass / silver alloy) so it must be some sort of bronze or blass alloy.
And you are probably correct about 406 and not 409. I am probably thinking about the C-17s and how they were renumbered. Take a look at the rods, they are also stamped with the original number. Pretty cool that these parts have survived 100 years!