WARNING: The following article contains minor spoilers for Marvel's Iron Fist Season 2, streaming now on Netflix.

The New York Police Department maintains a significant presence across Marvel's Netflix dramas, serving as a foil and occasional ally of the street-level heroes. But the NYPD also routinely delivers the expected Stan Lee cameo, with photos of his smiling Captain Irving Forbush adorning precinct walls and recruitment posters. Now, in Iron Fist Season 2, the police force adds to its Easter egg repertoire a far deeper nod to Marvel Comics: 616.

That number will mean little to the casual viewer, but comic book fans will immediately recognize it as the designation for the primary continuity of most Marvel Comics stories. A term used primarily by readers, Earth-616 is where the adventures of Spider-Man, the Avengers, Daredevil, the X-Men and the majority of other Marvel heroes take place. It's like DC's Earth-One, and just as nerdy.

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In the aptly titled fifth episode, "Heart of the Dragon," Colleen Wing (Jessica Henwick) and Misty Knight (Simone Missick) hit the streets in search of Danny Rand (Finn Jones), who's missing following what they suspect is an attack by the enigmatic Mary (Alice Eve). On a break from the NYPD while she decides whether to accept a promotion to captain, Misty encourages Colleen's burgeoning detective skills as they drive around in her unmarked car, searching for leads to Mary's identity and whereabouts. The radio squawks, alerting all units to multiple fatalities at a nearby nightclub, designated 616, which Misty informs Colleen is police code for "possible suspect with abilities."

It makes sense that, following run-ins with Jessica Jones, Luke Cage and Iron Fist, not to mention Bushmaster, Alisa Jones and whatever the hell they think happened at Midland Circle, that the NYPD would devise a code for incidents involving superhumans. And on a meta level, "616" is just about perfect.

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Introduced in 1983 in a Marvel UK story by Alan Moore and Alan Davis, the term Earth-616 was used to differentiate Captain Britain from the Marvel Comics Universe from the other members of the Captain Britain Corps, each of whom hails from an alternate universe. Although there are differing accounts as to why 616 was selected, the term found more common use among readers, and creators, after it was used in Marvel's Excalibur.

References to "616" have appeared in 2013's Thor: The Dark World and on Marvel's Agents of S.H.I.E.L.D., this is the first time the term as crept into the Netflix corner of the Marvel Cinematic Universe. However, it's almost certain not to be the last.

Available now on Netflix, Marvel’s Iron Fist Season 2 stars Finn Jones as Danny Rand, Jessica Henwick as Colleen Wing, Sacha Dhawan as Davos, Tom Pelphrey as Ward Meachum, Jessica Stroup as Joy Meachum, Simone Missick as Misty Knight and Alice Eve as Mary Walker.