Who is The Monarchs from Tales of Henry James: The Texts of the Stories, the Author on His Craft, Background and Criticism and what is their importance?

Major and Mrs. Monarch are supposedly "the real thing", born to gentile upper-class British life. They look the part. Both are tall, good-looking and have excellent posture. The major has a full moustache, and a hand-tailored suit. His wife is well-groomed and beautifully dressed. They are accustomed to visiting friends at their country houses. Now in need of money, they appeal to an illustrator to use them as models for his work, with unexpected results.