Answer:
The conversation contains implied information that becomes the final twist in the plot.
Explanation:
"Pretty young to hold an office like that, isn't he?" asked the other.
"Young!" exclaimed the first speaker, "why--Oh! didn't you catch on? Say--did you ever know an officer to handcuff a prisoner to his right hand?"
This conversation is really meant to tell you that Easton is the criminal not the Marshall and the Marshall was doing him a kindness not embarrassing him in from of this woman he knows. This is the plot twist