Title text:
In 1899, people were walking around shouting '23' at each other and laughing, and confused reporters were writing articles trying to figure out what it meant.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3184/
Additionally, while technically imbued with 'meaning', even the number 420 itself is somewhat meaningless and was originally used to delineate those who knew from those who don't. It's just that it got famous enough that we now almost all know.
In that sense I would argue it filled more or less the same function as 67.
I'd like to add that that's called a shibboleth :)
I’ve heard it said that 420 referred to the time 4:20 pm, when a group would come together to smoke, but that sounds contrived.
420 can also refer to the birth date of Adolf Hitler, which makes 420 a bit darker than just “haha, smoke.”