A reference to the film The Third Man. Terry says:
“It may be that there is a whole generation now to whom The Third Man is just a man after the second man. And after all, it wasn’t set in Vienna, Ohio, so it probably never got shown in the US:-)”
The book contains a couple of other resonances with The Third Man. In the film, the British, French, American and Russian occupation troops in Vienna patrol the city in groups of four, one from each country, to keep an eye on each other. Carrot sends the Watch out in similar squads of a human, a dwarf and a troll. The final chase through the sewers under the city also mirrors the film.