An inscription came into his mind which he had read on the gateway of the cemetery at Errancis where Saint-Just, Robespierre and their sixteen beheaded comrades lay buried. It consisted of one word:
Dormir --- to sleep.
Previously, I had thought: If I am ever in Paris again, I will go and see that. But then it occurred to me that there would surely be a picture on the Web. So I looked it up, and what did I find? Rubashov (and Koestler) could not have read any such thing. The cemetery no longer existed at the time of the events in the novel; in the mid 19th century the skeletons were dug up & moved to the catacombs, and the site was built over; all that remains there now is a commemorative plaque on one of the buildings. Also, it was not "the cemetery at Errancis"; Errancis was the name of the cemetery, not its location. Also, the inscription is supposed to have consisted of two words: "Dormir. Enfin!" (To sleep. At last!), and it is not certain that it existed.
Likewise, in his autobiographical volume The Invisible Writing, there is a substantial section on Bertolt Brecht (Koestler was living in Berlin in the years around 1930). It contains the following:
Brecht's greatest success, Die Dreigroschenoper, was a modernisation of Gay's Beggar's Opera.... One of its refrains, which compressed Brecht's message into a single formula, became a catchphrase in pre-Hitlerite Germany: 'First comes my belly, then your morality'. (Denn erst kommt das Fressen, und dann kommt die Moral.)The empty words "Denn" & "und" are not in the song. They do not affect the meaning, but they do spoil the meter, making it certain that Brecht was not actually remembering the tune when he wrote this. He remembered an impression, and filled in the facts.
I trust his impressions --- I imagine he can be relied on to describe the atmosphere of the many places & times he went thru, and most particularly the typical arguments people were having. But as to the telling details, you had better look them up.