In 1955, the curtain went down on one of the last great royal fairy tales. The heroine dropped the romantic ingenue role and took up the lead in a complicated psycological drama. The villain-hero got left behind on the ramparts, pondering the old values.

You remember the scandalous story of Princess Margaret, sister to the Queen of England, and Group Capt. Peter Townsend, Dashing Divorce. The wicked commoner captivated the heart of the young princess, who was then forced to renounce personal happiness to preserve the public morality.

We then had a quick change of eras. The princess married a commoner, after all, and took up commonly practiced standards of behavior. She fought with her husband in public, officially split from him and got herself a new boy friend. The dynasty has survived it all.

And what of Peter Townsend whose divorce was such a threat to the royal family he southt to enter? He is now 62 years old, silvery, handsome and charming.

On the personal side, he has been "very happily married" for 18 years to a Belgian, and says he doesn't even read about Princess Margaret.

Professionally, he continue to profit from the publicity of his old romance, but agonizes over his effort of "conform of standards of decency, discretion and humanity" while doing so.

Townsend has just published his fourth book, a study of Princess Margaret's father, George VI, whom he served as Deputy Master of the Household. The subtitle of "The Last Emperor" claims it is "an intimate account," but Townsend is quick to point out that it is not.

Acknowledging that he had not been asked to write about George VI because of any groundwell of interest in that monarch's blameless life, Townsend says he felt an obligation to produce a book which "maintained the proper respect. It it is admired for nothing else, it will keep to those standards. I would never let out any secret. After all, I was a trusted servant, if you will."

The son of a British high commissioner in Burma, Townsend fell in love with flying at an early age and had a distinguished Air Force career before he was tapped, "as a compliment to the fighting services," to be an equerry at Buckingham Palace.

"I went clean from the cockpit into the court," he said. Serving as a personal aide was "not a very big drain on one's intellectual capacity," he sais, "but one was close to the king and it was a very enriching experience."

At this point one expects secrets, intrigue, excitement, never-before-revealed information, gossip, intimacy.

Certainly not. If the royal family has turned relatively informal, Townsend has not.

Although he knows that "the royal family now invites in the television cameras," he refuses to perform a similar task. The most he will reveal is that behind the facade of the model king was a model husband and father.

In Townsend's description of the king's reaction to losing his ancestors' empire - the theme of the book - "the king certainly didn't remain indifferent to all those ghastly goings on, but I don't know what his inner thoughts were about himself. Never in my presence did he mop his brow and say, 'Oh, this is terrible.'"

Nor does Princess Margaret appear except in the most casual of references: "She had nothing to do with the unwinding of the empire."

And while he is now working on his autobiography, Townsend promises that "I'm not revealing any details" about his romance there, either. "I'm certainly not going to say that I saw her in the moonlight and there were tears in her eyes - none of that balderdash. But there are certain historical and constitutional aspects of the situation that might be interesting, and even amusing. The queen, as head of the state, which recognizes divorces, is in permanent contradiction with herself, as head of the church."

His theme for George VI is the "extraordinary grace" with which that king accepted the dissolution of the British Empire. "He was a very wise king to move ahead with the time. He didn't 'lost' the empire - he gave it back. And he didn't start hollering when it happened - he went quietly off to the hills. He was like a doctor who knows he's fought very hard to save a patient; whatever happens, he knows he's done his best."

Is it then possible that relaxed personal standards - which made the queen who rejected him accept her sister's separation - are a similar adaptation to the inevitablity of change? Perhaps, Townsend said. He is not sorry to have people know that "I don't have to go around with a bell on my neck because I'm contaminated." But with no royal house to uphold, he, at least, can indulge in consistency.