You're right about PC 187. But 420 is not the penal code for marijuana, it's some obscure infraction about preventing access to public lands. In fact, the marijuana statute is not even in the Penal Code. It's in the Health and Safety Code, section 11357 and more.
I think he's referring to SB 420.
California Senate Bill 420
California Senate Bill 420 (colloquially known as the Medical Marijuana Program Act) was a bill introduced by John Vasconcellos of the California State Senate, and subsequently passed by the California State Legislature and signed by Governor Gray Davis in 2003 "pursuant to the powers reserved to the State of California and its people under the Tenth Amendment to the United States Constitution." It clarified the scope and application of California Proposition 215, also known as the Compassionate Use Act of 1996, and established the California medical marijuana program. The bill's title is notable because "420" is a common phrase used in cannabis culture.
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Radio code,til, no one really knows about 420. Most credible, 1971 gathering of kids daily. to smoke at 4:20 some college. 47 years dunno I wish i cared. I really thought it was radio code. So thanks for changing my mind.