It is Stardate 866274.3, which places it, by TNG reckoning, in 3189. However, as I’ve noted before, this is an impossibility, since Burnham arrived in the 32nd Century in 3188, then spent a year before reuniting with Discovery (3189), then months passed between Seasons 3 and 4, and also between Seasons 4 and 5 so at a minimum it should be 3190. So stardates have to be working differently now.
The image Burnham is looking at is of the Progenitor hologram from TNG: “The Chase”, played by Salome Jens, who also played the recurring role of the Female Changeling in DS9.
The unwritten Starfleet code of conduct is apparently “Don’t snitch.” Rayner has been in Starfleet for 30 years, during which time the Federation was apparently at war (but no longer), and he’s fought side-by-side with Vance. The Breen are infighting over a new leader (they are now an imperium instead of a confederacy), and the Orions are regrouping, presumably after the splintering of the Emerald Chain at the end of Season 3 (DIS: “That Hope is You, Part 2”).
Book imitates Saru’s threat ganglia, which started as a response to sensing danger, but following his vahar’ai (DIS: “An Obol for Charon”), were replaced organs which could shoot spines.
The Promellians fought a long war against the Menthars which ended in the extinction of both species in the 14th Century. The Enterprise-D encountered a Promellian battle cruiser caught by a Menthar booby trap and nearly fell victim to the same trap in TNG: “Booby Trap” (2366). A necropolis is basically a very large graveyard or tomb, hence Burnham’s concerns about it being a sacred space.
Zareh was a courier who encountered Saru and Discovery when they first arrived in the 32nd Century (DIS: “Far From Home”), attempting to extort dilithium from them. He allied himself with Osyraa during her commandeering of Discovery and eventually died in the ship’s turbolift systems while fighting Book (DIS: “That Hope is You, Part 2”).
The Promellians’ use of Lang-cycle fusion engines was first mentioned in TNG: “Booby Trap”. I do wonder, however, what the statue was supposed to represent, since Promellians as seen in TNG only had two eyes, not four.
Being able to generate new phasers with just a metaphorical flick of the wrist is really handy.
Booker says that Moll and L’ak are Sui, couriers who take the most dangerous jobs fpr the action and latinum (from suicide, perhaps?).
The inscription starts with “Jolan tru, zarbalgon…" which Burnham translates as “Hello, wanderer”. Jolan tru is a traditional Romulan greeting which was used as both “hello” and “goodbye” (TNG: “Unification I”). Where Burnham learned Romulan is unexplained, but likely from her mother, a member of the Qowat Milat or on Ni’Var (DIS: “Unification III”). At the time she left the 23rd Century, the Romulans were still safely ensconced behind the Neutral Zone.
Burnham identifies the inscription as a Romulan revlav, which from context must be a poetic form, consisting of five verses or lines. “Hello, wanderer. Many worlds have you traveled. Opaline waters call to you. Thoughts are shared.” Saru says that seems to point to Betazed, but the fifth verse is missing.
Saru and Burnham have really been boning up on Romulans. The shaiqouin, the false front door of Romulan houses, was first mentioned in PIC: “The End is the Beginning”, but there was called a shaipouin. The last verse reads, “A world like no other, where two souls entwine, joined as one.” Adira deduces that the whole poem in context points to Trill.
Booker reminds us that his name is an alias, passed down from courier to courier (DIS: “That Hope is You, Part 2”, “Species Ten-C”).
Saru packs his knife, a gardening tool given to him by his sister on the night he left Kaminar to join Starfleet (ST: “The Brightest Star”).
Saru talks about giving oneself over “to the journey”, which reminds me of the toast that Barclay gave in the alternate future’s tenth anniversary of Voyager’s return (VOY: “Endgame”). Saru first warned Tilly about not touching the swamp kelp in bloom in DIS: “Choose to Live”.
Vance’s daughter previously appeared in DIS: “Kobayashi Maru”, but this is the first time we learn her name is Charlie (named after him, presumably).
Other receipients of the Grankite Order of Tactics (first mentioned in TOS: “Court Martial”) include James Kirk, Jean-Luc Picard (PIC: “Remembrance”) and Liam Shaw (PIC: “The Next Generation”). The latter three also received its Class of Excellence.