• The arms merchant whom we see Moll and L’ak dealing with appears to be an Annari, who were introduced in the VOY episode, “Nightingale”, which I mention only because I believe it is the first time we’ve seen a member of Delta Quadrant species in DIS, though not the first mention.
• The weapon he acquired for them is a Krenim “chronophage,” or time bug, which presumably would have originated in the Krenim Imperium, also in the Delta Quadrant, as seen in “Year of Hell” and “Year of Hell, Part II”. In those episodes, the Krenim also had a weapon that manipulated time.
• Rayner establishes that the time bugs are left over from the Temporal War, which was mentioned in “That Hope Is You, Part 1” as being the reason time travel is outlawed in the 32nd century.
• The Emerald Chain were the antagonist organization of season three of DIS.
• We see that the time bug is what Moll put on Adira’s uniform sleeve at the end of the previous episode, “Jinaal”.
• Burnham appears to keep either a copy of the Vulcan Kir’Shara or a similar artifact in her ready room.
• The opening credits sequence has changed to include both the parts of the key that the Discovery crew have secured being inserted into the ring.
• Burnham and Rayner find themselves being transported between multiple points in the USS Discovery’s existence.
• The Discovery following Burnham in the Red Angel suit to the future in “Such Sweet Sorrow, Part 2”.
• The construction of Discovery apparently in drydock in San Francisco. The first time we saw a starship being constructed at a ground facility was the Kelvin timeline USS Enterprise.
• Stardate 1051.8, which was the stardate Burnham recorded in her log in “Such Sweet Sorrow” at the beginning of the episode when they’re preparing to abandon Discovery and destroy it, somewhat before the battle with Control depicted here begins. That battle was in “Such Sweet Sorrow, Part 2”.
• Stardate 865422.4. Not a stardate previously given, but apparently during the Emerald Chain attack on Discovery in “There Is A Tide…”
• 27 years in the future.
• Some point after Discovery arrives in the 32nd century in “Far From Home”, but before the retrofit in “Scavengers”.
• After Burnham becomes captain in “That Hope is You, Part 2”, but before the destruction of Kwejian in “Kobayashi Maru”.
• Season one between “Context is For Kings” and “The Butcher’s Knife Cares Not For the Lamb’s Cry”.
• The hardhat worn by the technician working aboard the Discovery during construction has the 23rd century symbol for Starfleet’s operations division on it.
• ”Well, he lives outside of time because of his tardigrade DNA.” Obviously. Stamets spliced the DNA of the giant tardigrade with his own in “Choose Your Pain”, and we learned that allowed him to exist outside normal spacetime in the time loop episode, “Magic to Make the Sanest Man Go Mad”.
• The Temporal Prime Directive was first mentioned in “Future’s End, Part II”.
• Zora is listening to a rendition of “Que Sera, Sera”.
• ”Are you stuck in a time loop now right now, Stamets?” He’s not, but Stamets was the only one aware of being stuck in a time loop in “Magic to Make the Sanest Man Go Mad”.
• Burnham runs into her younger self. Other characters have met iterations of themselves via time travel in:
• “Yesteryear” - Spock
• “Time Squared” - Picard
• “Firstborn” - Alexander
• “Visionary” - O’Brien
• “Children of Time” - Odo; the Dax symbiont
• “Wrongs Darker Than Death or Night” - Kira
• “Time’s Orphan” - Molly
• “Relativity” - Seven of Nine
• “Endgame” - Janeway
• “E²” - T’Pol
• 2009’s “Star Trek” - Spock
• “A Quality of Mercy” - Pike
• It’s Airiam! From Star Trek! Despite Airiam’s appearance here taking place during season one, it is Airiam’s season two actor, Hannah Cheesman, under the prosthetics.
• It’s Bryce! From Star Trek! Ronnie Rowe Jr. reprises the role he played in seasons one, two and three, before leaving the regular cast, and appearing in only four episodes of season four.
• Burnham demonstrates her familiarity with Owo by mentioning the operations officer joined Starfleet because she wasn’t able to prevent a childhood friend’s death, something Owo told Saru in “Stormy Weather”.
• Burnham convinces Airiam she’s genuine by telling Airiam she sacrifices her life to save everyone else in “Project Daedalus”, an act that none of the rest of the crew believe Airiam would perform.
• ”You love ships, you love the Crossfield.” “Who doesn’t?” Buddy….
• Rayner has to stick a chroniton stabilizer into a field of accelerated time protecting the time bug with his bare hand for some reason, causing the appendage to age rapidly. Picard accidentally stuck his hand in “Timescape”.
"Who doesn't?" I mean asking if a Starfleet officer likes starships is kind of like asking of a /c/star_trek browser likes Star Trek.