My annotation has an entry by several literary critics and biographers on The Letter. I'd like to highlight a few that struck me.

Stuart Tave notes that one way of summarizing the action of Persuasion is "to say that it begins when Anne's word has no weight and it ends when her word pierces a man's soul."

I like this a lot. "Only Anne," the non-entity has become "the only one."

Mary Favret explores the interplay between what Anne is saying and what Wentworth is writing: "Wentworth's letter literally hangs on Anne's every word. His message is governed by her expressions: the full meaning of his 'half'-way language required the recipient's remembering her concurrent discussion with Captain Harville. … Like Anne's consciousness, Wentworth's letter defies limits and devours the words in the air about it. As the stories of Benwick, Harville's sister, and the Musgrove daughters fade away, the letter affirms and intensifies the language, the unique understanding, shared by Anne and Wentworth."

I like that image of the letter "devouring the words in the air" creating almost a vacuum, a bubble of space enclosing Anne and Frederick, shutting out all others.

For Tony Tanner, the most important moment in the scene is when Wentworth drops his pen. … "a 'slight noise' draws their attention to Wentworth. 'It was nothing more than that his pen had fallen down.' Nothing more - in many ways it is the most quietly dramatic and loaded incident in the book. The pen may, generally speaking, be in 'their hands'; but at this crucial moment the pen - a specific one - had dropped from his specific - hand. However unintentionally, however momentarily, he is disproving the generalization which Anne is enunciating … Wentworth at this critical moment has … dropped … that instrument which is at once a tool and a symbol of man's dominance over women; the means by which they rule women's destinies, literally write … their lives. It is as if he is open to a more equal (unscripted) relationship in which the old patterns of dominance and deference are abandoned, deleted - dropped."

I like this as well - Anne saying that "the pen has been in their hand" writing histories while the pen is now in Frederick's hand as he tries to change their future history.