Here's a precursor to "420" as marijuana slang, fully 56 years earlier than its supposed origin in 1971.  The text reads, "Cabbage soup is made in a lidded pot.  Yes, but the lidded pot falls into the cabbage."  "Lid," "pot," "cabbage," and "420" are all euphemisms for cannabis.  From 1915, printed in an issue of Le Rire the following year.

Do you detect a coded meaning to this ad from the turn of the century?  Though early for the use of "Mary Jane" as slang for marijuana, the "Maria Jane" reference here is paired with the unusual name "Blifkins" ("bliff" rhyming with "spliff," suggesting a cigarette of cannabis and tobacco), in the psychedelic context of "a change [coming] o'er his dreams."  When we include the hallucinatory portrait in which two figures emerge from the man's face, the ad seems to be suggesting that this particular tobacco combines happily with cannabis.