In a South London photographic studio, the music was pumping.

French singer Dalida was playing. The woman in front of the camera was tossing her hair back and laughing, the energy in the room, palpable.

Those shots, taken in the spring of 1997 by internationally-renowned snapper Mario Testino would be published on the cover of Vanity Fair in June.

They would instantly cause a global sensation: Here was the newly divorced Diana, Princess of Wales, like she had never been seen before.

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Sexy, carefree and happy, a stark contrast to the woman who only two years before had sat down with Martin Bashir for a TV interview, her voice shaky and her eyes wide.

The Testino shoot and the accompanying piece written by high-profile journalist Cathy Horyn were putatively to promote the charity auction of 79 dresses Diana was about to hold at Christies.

But really, those shots were something more powerful — they were a declaration of independence.

Photographer Mario Testino stands by one of the images taken five months before Diana's death in Paris. Photo / Supplied
Photographer Mario Testino stands by one of the images taken five months before Diana's death in Paris. Photo / Supplied

"The looks she throws at Mario are so warm and confidential they may be her most revealing statements since the divorce," Horyn wrote at the time.

The now iconic images would be her most intimate shoot ever and the last official portraits ever taken of her.

The issue of the magazine was set to come out nearly a year after her divorce from Prince Charles had been finalised in August 1996. As Horyn points out, the Princess had lost not only a husband but her position.

(That same month she lost the right to style herself as HRH.)

Now, months later, Diana was carving out a new identity for herself as a global advocate and ambassador, flying to Angola to highlight the heinous violence of landmines and then later, war-torn Sarajevo.

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"It was a new period in her life, you could feel an energy between her joy or happiness," Testino has said. "I wanted to show her joy and excitement."

Testino's photos on display before her death in Paris in 1997. Photo / Supplied
Testino's photos on display before her death in Paris in 1997. Photo / Supplied

Now, more than 20 years after they were taken at that South London studio, they remain some of the most famous images of perhaps the most famous woman of the 20th century.

Which makes her sons' relationship with them all the more heartbreaking.

Both Prince William and Prince Harry, according to reports, have been known to carry a framed copy of a shot of their mother by Testino from that day with them when they travel.

It is a haunting image: Grown men carefully packing and moving a copy of one of Testino's images of the Princess, so that they could have her beaming, joyful presence by their side wherever they were in the world.

The fact that of all the formal portraits and relaxed family snaps they could choose from that they opted for a photo from this series is telling. That they would want to remember her as the world saw her in that moment: On the precipice of something truly great and more importantly, finally finding real joy and contentment.

What makes Harry and Wills' attachment to these images even more poignant is that they are reminders of the fact Diana died right at the point she was finally achieving that most elusive of quarries, true happiness, after decades of heartbreak and rejection.

These shots are not only touching moments, they are a heart-rending symbol of what could have been for Diana and her beloved boys.

In April of 1997, Diana had taken the boys to Barbuda in the Caribbean. Photos of her from that trip show her smiling and relaxed in the sunshine.

Then, in July, she again took her sons on holiday to St Tropez, staying at Mohamed Al-Fayed's 30-room villa. When she went on vacation again it was on a cruise of the Mediterranean with Dodi Al-Fayed aboard his father's superyacht, the Jonikal.

Harry and Wills were, meanwhile, at Balmoral Castle in Scotland with their father and grandmother. When their mother rang on the 31st of August they were having a ball playing with their cousins.

"Harry and I were in a desperate rush to say goodbye, you know, 'see you later'," William said in a 2017 documentary about the Princess. Harry recalled: "It was her speaking from Paris. I can't really necessarily remember what I said but all I do remember is probably regretting for the rest of my life how short the phone call was," he said.

What the Testino photo anecdote betrays is the wounds and the incredible loss the boys have carried for years. The psychological toll her death had on these now grown men is part of the reason they, along with Kate, Duchess of Cambridge; launched their Heads Together mental health charity.

The Princess once said of her boys, "I want them to lead from the heart."

With Diana's beaming face always with them, they are truly honouring her wish.