D. Scott Chatfield
I think a UN committee came up with the numbers starting in 1975, but they didn't appear on freight cars until mid-1983. That's when I saw my first one (1203, gasoline) on a company service trailer and asked "what's this?" Somebody higher up at the Southern decided that maybe a manager with my duties (overseeing proper handling and marking of hazmat in piggyback trailers, among other duties) might need to know about these newfangled UN codes. And I was issued a copy of the federal Bureau Of Explosives tariff, which listed ALL of the codes.
That said, the Tichy sheet includes several generic color-coded hazmat placards that date back, I think, to the late '60s. Still too late for this list.
I think Microscale makes a sheet that includes late Steam Era diamond-shaped placards, which were mostly white with a mix if red and black lettering.
Which brings up the question, when did the diamond-shaped placards first appear? Union Tank had their distinctive square holders on all of their tanks by 1940, didn't they? I made a couple sets from plastic strip and Plano frames not too long ago for some UTLX tanks I made for a friend, because no one can have too many black tank cars.....