Tank car dangerous goods warning placards in the 1950s?


I'm sure this must have been covered before ( I couldn't find it in
Yahoo search mode), but while constructing some Proto 8K gallon tank
cars I began to wonder when the warning placards used for hazardous
substances began to be used on these cars. Would these be present on
tancars during the early to mid 1950s?
Thanks in advance
Stefan Lerché
Duncan BC Canada

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