Fans love it when a TV series throws in a line or an action that is only understood by faithful viewers, those who have been watching since season one. They are callbacks, running gags, internal references, brick jokes, slow burns, in-jokes, or Chekhov's gags. You could look all those up at TV Tropes, but then you can kiss your evening goodbye. In the above case, a subject comes up 43 years later, and the internal reference is just dialogue for most viewers, but pure gold for those who remember the earlier episode. Below, the gun going off for no reason is just a random fact, and the reason it was mentioned at all only becomes evident years later.
Maybe these things are in-jokes for the writers, or maybe they are put in just to see if anyone notices. Even if just a few fans do, you can bet they'll share the cleverness on social media tomorrow. See 13 such references in a pictofacts article at Cracked.