Charles Spencer Shares Rare Childhood Photo with Sister Princess Diana and Their Mom

"I love how happy each of us looks," Charles wrote in his caption

Charles Spencer is sharing a rare glimpse into his childhood with Princess Diana.

In a post on Instagram Saturday, the 9th Earl Spencer, 59, shared a black-and-white snapshot of himself and Diana smiling together in the late '60s.

The photo shows a young Charles posing on a swing next to Diana as their mother, Frances Shand Kydd, held onto him from behind.

“My mother, Diana and I, in c. 1967,” Charles — who has a new memoir about his childhood and experience in boarding school called A Very Private School, out March 12 — wrote in the caption. “I would have been 3, and Diana 5 or 6.”

“I love how happy each of us looks,” he added of the sweet family shot.

Related: Princess Diana's Siblings: Everything to Know About Her Brother and Sisters

Also in the caption, Charles shared a tidbit about his childhood nickname.

“At this stage of my life, my mother nicknamed me 'Buzz' - because she saw me as having the endless energy of a rather happy - and busy - bee,” he wrote, adding several hashtags, including “#Family” and “#SpencerFamily.”

Diana and Charles were two of five children born to Frances and John Spencer. They grew up with two older sisters, Lady Sarah McCorquodale and Lady Jane Fellowes, and also had an older brother, John Spencer, who died shortly after his 1960 birth.

Sarah and Jane have lived relatively private lives since Diana’s death in 1997, while Charles, an author, historian and podcast host, has been a vocal protector of Diana's legacy.

Related: Charles Spencer Visits Sister Princess Diana's Private Memorial: 'A Special Corner of Althorp'

While speaking with PEOPLE in 2017 two decades after Diana’s death, he revealed that there are "many myths" about he and his siblings’ upbringing — including that they never called his sister "Di" and that the royal was never particularly shy.

"First of all, none of us ever called her 'Di' at home. In fact, there are so many myths from our childhood that are just so ridiculous. That's one of them," he told PEOPLE at the time.

"I just think she was never shy, but she was canny about people and she was reserved to start with," he added. "And she would take a judgment of somebody before reacting to them. So, that's not shy ... that's actually quite clever."

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In the same interview, Charles also told PEOPLE that he has always felt "intensely protective" toward Diana, and wishes he could have saved her from her tragic accident.

“I was furious, I wasn’t just angry,” he recalled at the time. “[I thought] what could I have done. It was just ... it was devastating.”

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