What detail from the story "After Twenty Years" by O. Henry contains a historical reference showing that the setting of the story is in the 1800s?
In O. Henry's story "After Twenty Years," the Chicago Police Department sent a telegram to the New York Police Department stating that 'Silky' Bob was wanted for questioning. According to the Encyclopedia of Chicago article titled "Telegraph" available on the Internet:
The telegraph, which received its first practical demonstration in 1844, came to Chicago in 1848. Telegraphy made possible instant communication with the East Coast, and eventually with the entire country.
When the plainclothes reveals his identity to Bob in front of the electrically-lighted window of the drugstore, he says:
“You've been under arrest for ten minutes, ‘Silky’ Bob. Chicago thinks you may have dropped over our way and wires us she wants to have a chat with you."
The word "wires" is a detail that contains a historical reference showing that the setting of the story is in the 1800s. Chicago might not have known Bob's full name but only that he was known as 'Silky' Bob. In those days it was impossible to transmit a drawing or photograph of a person by wire. Messages could only be sent by Morris Code, consisting of dots and dashes, that is, short beeps and longer beeps, with each combination of beeps representing a letter of the alphabet. The Chicago Police would have sent a description of the wanted man with emphasis on any distinguishing characteristics or identification clues. When Bob lights his cigar in the doorway of the hardware store, Jimmy Wells, now a policeman, can see two distinguishing clues described in the telegram.
The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a little white scar near his right eyebrow. His scarf pin was a large diamond, oddly set.
The wire from Chicago would have mentioned the little white scar near his right eyebrow and would have given a more detailed description of the scarf pin with an oddly set large diamond. The scarf pin might have been surrounded by little rubies, for example. In any case, it would be one of a kind and would identify 'Silky' Bob unmistakably. That was why O. Henry put these details in his description of the wanted man. The author realized that the New York Police could not have received any kind of photo or sketch of Bob by wire.
While they are talking, Jimmy sees another identifying detail.
The waiting man pulled out a handsome watch, the lids of it set with small diamonds.
The wire from Chicago would have contained the information that the wanted man had a watch with the lids set with small diamonds.
Bob tells Jimmy, whom he doesn't recognize as his old friend:
"I came a thousand miles to stand in this door tonight, and it's worth it if my old partner turns up.”
A thousand miles is about the distance from Chicago to New York, and there would be no other important city a thousand miles west of New York. So the fact that Bob must have recently arrived from Chicago is further proof that he is the man wanted by the Chicago police.
When the plainclothes man arrives twenty minutes after Jimmy's departure, he asks leading questions to get Bob to identify himself. And then he gets Bob talking about his exploits in the West and making it unmistakably clear that he is the man the Chicago Police are after.
The two men started up the street, arm in arm. The man from the West, his egotism enlarged by success, was beginning to outline the history of his career. The other, submerged in his overcoat, listened with interest.
Even though the Chicago Police could not send a picture of 'Silky' Bob by wire, they were able to send enough information to enable Jimmy Wells to recognize his old friend as the wanted man. Nowadays, perfect photographs can be transmitted by wireless electronic means, but these did not exist in O. Henry's horse-and-buggy days.