ValMu fields a call from "Lane at SCDP" in New York. SCDP refers to in Sterling Cooper Draper Pryce, the New York City advertising agency from the American TV show Mad Men, which was transpiring in the same time frame. "Lane" was Lane Pryce, the English advertising executive played on Mad Men by Jared Harris.
During the closing credits, certain letters in the names are red. These letters spell out "Venerable Bede," the name of the English monk who wrote the Historia Ecclesiastica Gentis Anglorum, which is mentioned by Morse when he corrects a statement Dr. Matthew Copley-Barnes makes.
In addition to featuring, as a major character, the arrogant academic Matthew Copley-Barnes, who also features in the Inspector Morse (1987) story The Infernal Serpent (1990), set some 24 years later, this story also mentions his wife and her career as a music teacher (an important detail in the plot of The Infernal Serpent (1990)) and briefly features his daughter Imogen. Here, Imogen is a young girl playing on the lawn; in The Infernal Serpent (1990), she is a badly-damaged adult played by Irene Richard and a key figure in the story.
A sign in the museum informs us that the missing trove is usually housed in "the A.L. Macullen Room". The name "A.L. Macullen" is a punning reference to that of Alma Cullen, the TV writer whose credits include four episodes of "Inspector Morse" - including "The Infernal Serpent", the story in which retribution (of a kind) is finally visited upon Dr. Copley-Barnes.
With obvious pride, Tony Frisco tells Morse that he has recently met Simon Dee, the latter asking him if he "admired his shirt". At the time this story is set, Simon Dee was enjoying his brief period of fame as a disc-jockey and was about to start his career as a talk-show host. The "shirt" reference may allude to Dee's acting debut (actually in 1969) as a trendy shirt-maker in "The Italian Job". Dee's popularity on British television was relatively brief, and his career collapsed more or less totally after a move to ITV in 1970. All attempts to resuscitate his broadcasting career in subsequent decades failed miserably and he died largely forgotten in 2009. He is the the obvious basis for the "Julian Calendar" character who has appeared in two "Endeavour" segments.
Colin Dexter: Sitting on a park bench watching Morse and Thursday walk by after they have investigated the scene of the trove heist.