King Charles III is thanking the public for their condolences as he continues to mourn the loss of his mother.

Queen Elizabeth II, Britain’s longest-reigning monarch, passed away on Sept. 8 at Scotland’s Balmoral Castle. She was 96.

The new king shared a heartfelt childhood photo of himself and the late queen in letters he sent out to supporters, thanking them for their sympathies. One recipient took to Twitter on Wednesday and tweeted a photo of the card, which was stamped with the new monarch’s cipher. It featured a photo of Charles and his mother at Balmoral Castle in 1952.

Charles, who was 3 years old at the time of the photo, is seen happily peeking out a window as his mother smiles over his shoulder.

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Prince Charles and his mother Princess Elizabeth

Prince Charles and his mother Princess Elizabeth looking out of a window at Balmoral in September 1952. The matriarch was officially crowned Queen Elizabeth II on June 2, 1953, at age 27. (Photo by Lisa Sheridan/Studio Lisa/Hulton Archive/Getty Images)

"It was so very kind of you to send me such a wonderfully generous message following the death of my beloved mother," Charles wrote. "Your most thoughtful words are enormously comforting, and I cannot tell you how deeply they are appreciated at this time of immense sorrow."

The king signed the letter as "Charles R," referring to his royal rank. The "R" stands for "Rex," which means "King" in Latin. It is the traditional signature for the monarch dating back to the 12th century. When used by queens, the "R" stands for "Regina," or "Queen in Latin."

Buckingham Palace previously announced that it had received over 50,000 letters, including 6,500 in just one day following the queen’s funeral on Sept. 19. Before the queen’s death, the palace received about 1,000 letters a week.

The palace noted that all letters are read. Responses are sent as the team processes thousands of items.

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Buckingham Palace Letters

In this image released on Sept. 30, letters of condolence sent by members of the public to King Charles III, the queen consort and the royal family are sorted by the Correspondence Team at Buckingham Palace. Since the death of Queen Elizabeth II, over 50,000 letters and messages of condolence have been sent to Buckingham Palace, including 6,500 in just one day after the Queen's state funeral.  (Photo by Victoria Jones - Pool/Getty Images)

Charles waited longer than any other heir to take the throne, and in many ways, embodies the modernization of the monarchy. He was the first monarch not educated at home, the first to earn a university degree and the first to grow up in the ever-intensifying glare of the media as deference to royalty faded.

He has been lauded as an early advocate of the environmental movement and won praise for working to improve the lives of young people in underprivileged communities.

However, he also has a reputation as a somewhat stuffy older man who is more at home on the polo field or one of his country estates than in modern Britain. Charles also alienated many people with his messy divorce from the much-loved Princess Diana and by straining the rules that bar royals from intervening in public affairs, wading into debates on environmental protection and architectural preservation.

As the U.K. mourned his mother, it quickly became clear that Charles was ready to be a more personal monarch. He has made a point of wading into the crowds of well-wishers, stopping to shake hands and exchange a few words, more like a U.S. presidential candidate appealing for votes than a king who inherited the crown from a line of ancestors stretching back to 1066.

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King Charles greets the public

King Charles III greets members of the public after an official council meeting at the City Chambers in Dunfermline, Fife, to formally mark the conferral of city status on the former town on Oct. 3, 2022, in Dunfermline, Scotland. (Photo by Andrew Milligan - Pool/Getty Images)

One woman even kissed him — a level of familiarity no one would have dared with Elizabeth.

The question is whether Charles will follow his mother’s example and muffle his personal opinions now that he is king, or use his new platform to reach a broader audience.

"My life will, of course, change as I take up my new responsibilities," Charles said. "It will no longer be possible for me to give so much of my time and energies to the charities and issues for which I care so deeply. But I know this important work will go on in the trusted hands of others."

The Associated Press contributed to this report.