Interesting info, Richard.
I'm sure glad I've never said "never" about material used for numerals.
I'd almost bet that if there was a metal fabricated "9" on the cabsides that it had been there for a very long time, maybe 1910.
It had to have been removed in 1939 when she was gussied up in "Q" livery for the World's Fair. She sported a twelve inch "9" for the fair, and a standard cast "Q" number plate with the "9" cast in the center.
There is probably a pair of 18" nines somewhere in somebody's attic in Federal Heights that were dragged home and forgotten.
Keep up posted on your findings!